Cellulose ester film, its manufacturing method, optical retardation film, optical compensation sheet, elliptic polarizing plate, and image display

ABSTRACT

A manufacturing method of a cellulose ester film satisfying the relationships, Nx&gt;Ny&gt;Nz, and 0.8≦R t /R 0 ≦3.5, the method comprising casting a cellulose ester dope containing a solvent of a good solvent and a poor solvent on a support to form a web; peeling the web from the support; transporting the peeled web (step D0); holding the edges in the transverse direction of the web (step A); stretching the resulting web in the transverse direction while applying a tension; reducing the tension in the transverse direction of the web; and drying the stretched web, wherein the residual solvent content of the web is 5 to 90% by weight at the beginning in the step A, and the residual poor solvent content in the residual solvent content of the web is from 15 to 95% by weight at the terminal point in the Step D0.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of U.S. application Ser. No. 10/154,188filed May 23, 2002 now U.S. Pat. No. 6,814,914.

FIELD OF THE INVENTION

The present invention relates to a cellulose ester film and itsmanufacturing method, and an optical retardation film, an opticalcompensation sheet, an elliptic polarizing plate, and an image display.

BACKGROUND OF THE INVENTION

Recently, a personal computer has been adapted for multi-media, andcolor display has been generally used in a laptop type personalcomputer. In the laptop type personal computer or a desktop monitor, anSTN type liquid crystal display or a TFT type liquid crystal display ismainly employed. In recent years, as the size of the liquid crystaldisplay is increased, the TFT type liquid crystal display, which has anexcellent image quality, has been predominated. Therefore, improvementof a viewing angle property has been eagerly required.

As a displaying mode of the TFT type liquid crystal display, an IPS modeor a vertical alignment (VA) mode is proposed and applied in addition toa conventional TN mode. A TN mode TFT liquid crystal display hascharacteristics in that cost of manufacture is reduced and efficiency oflight utilization is high due to its optical rotatory mode. An opticalcompensation sheet developed-in recent years improves a viewing angleproperty and is widely used.

For example, an elliptic polarizing plate for viewing angle compensationhas a multilayer structure, and a typical example thereof has astructure a first transparent support (ordinarily, a cellulosetriacetate film)/a polarizer/a second transparent support (ordinarily, acellulose ester film)/an optically anisotropic layer formed by fixingthe orientation direction of a liquid crystal compound in a specificdirection/a transparent support C (for example, a stretched celluloseester film) having an optically biaxial property, wherein the structureis obtained by forming the optically anisotropic layer directly orindirectly on the second transparent support, and then forming thetransparent support C having an optically biaxial property on theoptically anisotropic layer.

The above structure is obtained by providing the optically anisotropiclayer formed by fixing the orientation direction of a liquid crystalcompound in a specific direction on the transparent support C.

As the first and second transparent supports, a cellulose triacetatefilm with a thickness from 40 to 80 μm manufactured according to asolution cast film manufacturing method is employed. A polyvinyl alcoholfilm doped with iodine and stretched is employed as a polarizer, and thepolarizer is inserted between the first and second transparent supportsto obtain a polarizing plate.

The optically anisotropic layer is formed, for example, by providing alayer containing a liquid crystal compound having a polymerizable groupon a support given an orientation capability through rubbing treatmentand fixing the orientation direction by hardening the layer by UV lightirradiation. The support is a transparent support having an opticallybiaxial property, and preferably a stretched cellulose ester film havinga thickness of 40 to 150 μm.

A cellulose ester film, which when continuously manufactured in view ofproductive efficiency, is stretched in the transverse direction, ispreferably employed as a transparent support having an optically biaxialproperty.

As a method of stretching the film in the transverse direction, a tenteraccording to a tenter method is generally employed. In the tenter, clipsare arranged at equal intervals and molecules are oriented only in thetransverse direction, and there is neither orientation in thelongitudinal direction of the molecules nor dimensional variation in thelongitudinal direction. As a result, the film thickness is reduced ininverse proportional to the stretching magnitude, resulting in anincrease of a retardation in the thickness direction. The dimension inthe longitudinal direction is regulated and orientation of the moleculesin the longitudinal direction of the film is also regulated. Instretching the film in the transverse direction, there is problem inthat molecules are oriented in the transverse direction, and acontractive force is generated in the transverse direction, whichincreases a plane orientation of the molecules and results in anincrease of a retardation in the thickness direction (R_(t)).

In a method of stretching in the transverse direction other than thetenter method above, there is also problem in that unintended stretchingin the longitudinal direction excessively increases a retardation in thethickness direction as compared with a retardation in the plane of thefilm.

As a film used in an LCD display, it is important that the film has notonly physical uniformity but optical uniformity. A film having a lowuniformity of a retardation in the plane or a retardation in thethickness direction or a low uniformity of dispersion of an opticallydelayed phase axis (hereinafter referred to also as an orientationangle) causes a light leak at black display, resulting in a seriousdefect practically.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a manufacturing methodof a cellulose ester film having excellent optical properties, thecellulose ester film manufactured according to the method, and anoptical retardation film, an elliptic polarizing plate, an opticalcompensation sheet and an image display each employing the celluloseester film.

Another object of the present invention is to provide a manufacturingmethod of a cellulose ester film having physical and optical uniformity,the cellulose ester film suitable for a member of an image display, andan optical retardation film, an elliptic polarizing plate, and anoptical compensation sheet each employing the cellulose ester film, andan image display employing the cellulose ester film, the opticalretardation film, the elliptic polarizing plate, or the opticalcompensation sheet.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 shows an illustration for explaining a stretching angle at whichthe web is stretched in the step B.

FIG. 2 shows one embodiment of a tenter step used in the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be attained by the following constitutions:

1. A method of manufacturing a cellulose ester film, the cellulose esterfilm satisfying the following formula:Nx>Ny>Nz, 0.8≦R _(t) /R ₀≦3.5, R ₀=(Nx−Ny)×d, andR _(t)={(Nx+Ny)/2−Nz}×d,wherein Nx represents the refractive index in the transverse directionin the plane of the film, Ny represents the refractive index in thelongitudinal direction in the plane of the film, Nz represents therefractive index in the thickness direction of the film, R₀ represents aretardation in the plane of the film, R_(t) represents a retardation inthe thickness direction of the film, and d represents the thickness (nm)of the film; the method comprising the steps of:

-   casting a cellulose ester dope containing a solvent of a good    solvent and a poor solvent on a support to form a web;-   peeling the web from the support;-   transporting the peeled web (Step D0);-   holding the edges in the transverse direction of the transported web    (Step A);-   stretching the resulting web in the transverse direction while    applying a tension (Step B);-   reducing the tension in the transverse direction of the web (Step    C); and-   drying the stretched web (Step D1),-   wherein the residual solvent content of the web represented by the    following formula (1) is 5 to 90% by weight at the beginning in the    step A,    Residual solvent content of the web (wt %)={(M−N)/N}×100  formula    (1)    wherein M represents the weight of the web to be measured, and N    represents the weight of the web after the web to have been measured    is dried at 110° C. for 3 hours, and wherein the residual poor    solvent content in the residual solvent content of the web    represented by the following formula (2) is from 15 to 95% by weight    at the terminal point in the Step D,    Residual poor solvent content (%) in the residual solvent content of    the web={weight of the poor solvent in the residual solvent of the    web/(weight of the poor solvent in the residual organic solvent of    the web+weight of the good solvent in the residual solvent of the    web)}×100.  formula (2)

2. The method of item 1, wherein the transporting tension of the film inthe step D0 is in the range of from 30 to 300 N/m.

3. The method of item 1, wherein the temperature of the film at thebeginning of the Step B is 30 to 140° C., the temperature of the film atthe terminal point of the Step B is 70 to 140° C., and the followingrelationship is satisfied:0.4×B0≦B1≦0.8×B0wherein B0 (%) represents the residual solvent content of the web at thebeginning of the Step B, and is 10 to 90% by weight, and B1 (%)represents the residual solvent content of the web at the terminal pointof the Step B.

4. The method of item 3, wherein the residual poor solvent content inthe residual solvent content of the web is from 15 to 95% by weight atthe terminal point of the Step B.

5. The method of item 1, wherein the temperature of the web at thebeginning of the Step B is 30 to 130° C., the temperature of the web atthe terminal point of the Step B is 60 to 130° C., and the followingrelationship is satisfied,0.8×B0≦B1≦0.99×B0wherein B0 (%) represents the residual solvent content of the web at thebeginning of the Step B, and is 10 to 90% by weight, and B1 (%)represents the residual solvent content of the web at the terminal pointof the Step B.

6. The method of item 5, wherein the residual poor solvent content inthe residual solvent content of the web is from 15 to 95% by weight atthe terminal point of the Step B.

7. The method of item 1, wherein the ambient temperature for the web inthe step B is 110 to 140° C., and the following relationship issatisfied,0.4×B0≦B1≦0.8×B0wherein B0 (%) represents the residual solvent content of the web at thebeginning of the Step B, and B1 (%) represents the residual solventcontent of the web at the terminal point of the Step B.

8. The method of item 7, wherein the residual poor solvent content inthe residual organic solvent content of the web is from 15 to 95% byweight at the terminal point of the Step B.

9. The method of item 1, wherein the ambient temperature for the web inthe step B is 30 to 130° C., and the following relationship issatisfied,0.8×B0≦B1≦0.99×B0wherein B0 (%) represents the residual solvent content of the web at thebeginning of the Step B, and B1 (%) represents the residual solventcontent of the web at the terminal point of the Step B.

10. The method of item 9, wherein the residual poor solvent content inthe residual organic solvent content of the web is from 15 to 95% byweight at the terminal point of the Step B.

11. The method of item 1, wherein the step B is carried out at astretching speed of from 50%/min to 500%/min, the stretching speed beingrepresented by the following formula (3):Stretching speed (%/min)={(web length in the transverse direction afterstretching/web length in the transverse direction beforestretching)−1}×100/time (min) necessary to stretch.  formula (3)

12. The method of item 1, wherein the good solvent vapor concentrationof the ambient air in the steps A and B is from 2000 ppm to less thanamount providing the saturated vapor pressure of the good solvent.

13. The method of item 1, wherein the stretching magnification in thestep B is from 1.1 to 2.5.

14. The method of item 1, wherein drying at the step D1 is carried outunder the condition of DB<DD1, wherein DB represents modulus ofelasticity in the longitudinal direction of the web at the terminalpoint of the Step B, and DD1 represents modulus of elasticity in thelongitudinal direction of the web at the beginning of the Step D1.

15. The method of item 1, wherein the edges in the transverse directionof the web are cut off with a slitter before the Step B.

16. The method of item 1, wherein a neutral zone is provided betweensteps A and B and/or between the steps B and C.

17. The method of item 1, wherein the cellulose ester film has a ratioHtd/Hmd falling within the range of from 0.62 to 1.0 in which Htd andHmd represent a tear strength in the transverse direction of the filmand a tear strength in the longitudinal direction of the film,respectively.

18. The method of item 1, wherein the cellulose ester film has an Stdfalling within the range of from −0.4 to 4.0% and an Smd falling withinthe range of from −0.4 to 4.0%, in which the Std and Smd represent arate of dimensional variation in the transverse direction of the filmbetween the films before and after having been subjected to heat andhumidity treatment at 60° C. and 90% RH for 24 hours and a rate ofdimensional variation in the longitudinal direction of the film betweenthe films before and after having been subjected to heat and humiditytreatment at 60° C. and 90% RH for 24 hours, respectively.

19. The method of item 1, wherein G2/G1 falls within the range of from0.9 to 1.0, in which G1 and G2 represent the plasticizer content of thedope at the casting step, and the plasticizer content of the web at theterminal point of the Step D1, respectively.

20. The method of item 1, wherein the thickness dispersion R of thecellulose ester film falls within the range of from 0 to 8%, R beingrepresented by formula (4):R(%)=(Rmax−Rmin)×100/Rave,   formula (4)wherein Rmax, Rmin, and Rave represent the maximum thickness in thetransverse direction of the film, the minimum thickness in thetransverse direction of the film, and the average thickness in thetransverse direction of the film, respectively.

21. The method of item 1, wherein the dispersion of the orientationangle of the cellulose ester film falls within the direction inclined atan angle of 90°±1° to the longitudinal direction.

22. The method of item 1, wherein the coefficient of variation of theretardation in the plane (R₀) of the cellulose ester film is not morethan 5%.

23. The method of item 1, wherein the coefficient of variation of theretardation in the thickness direction (R_(t)) of the cellulose esterfilm is not more than 10%.

24. The method of item 1, wherein the haze of the cellulose ester filmis in the range of from 0 to 2%.

25. The method of item 1, wherein the total acyl substitution degree ofthe cellulose ester film is from 2.3 to 2.85, and the acetylsubstitution degree of the cellulose ester film is from 1.4 to 2.85.

101. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:Nx>Ny>Nz, 0.8≦R _(t) /R ₀≦3.5, R ₀=(Nx−Ny)×d, and R_(t)={(Nx+Ny)/2−Nz}×d,wherein Nx represents the refractive index in the transverse directionin the plane of the film, Ny represents the refractive index in thelongitudinal direction in the plane of the film, Nz represents therefractive index in the thickness direction of the film, R₀ represents aretardation in the plane of the film, R_(t) represents a retardation inthe thickness direction of the film, and d represents the thickness (nm)of the film; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   transporting the peeled web (Step D0);-   holding the edges in the transverse direction of the web (Step A);    and-   stretching the held web in the transverse direction (Step B), the    residual solvent content of the web represented by the following    formula (1) being 5 to 90% by weight at the beginning of the    stretching;    Residual solvent content of the web={(M−N)/N}×100  formula (1)    wherein M represents the weight of the web when measured, and N    represents the weight of the web after the web measured is dried at    110° C. for 3 hours.

102. The method of item 101, wherein the transporting tension of the webin the step D0 is in the range of from 30 to 300 N/m.

103. The method of item 102, wherein the residual poor solvent contentin the residual solvent content of the web represented by the followingformula (2) is from 15 to 95% by weight at the terminal point of theStep D0:Residual poor solvent content (%) in the residual solvent content of theweb={weight of the poor solvent in the residual solvent of theweb/(weight of the poor solvent in the residual solvent of theweb+weight of the good solvent in the residual solvent of theweb)}×100  formula (2)

104. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction (Step B), the    temperature of the web at the beginning of the Step B being 30 to    140° C., the temperature of the web at the terminal point of the    Step B being 70 to 140° C., and the following relationship being    satisfied:    0.4×B0≦B1≦0.8×B0    wherein B0 (%) represents the residual solvent content at the    beginning of the Step B of the web, and is 10 to 90% by weight, and    B1 (%) represents the residual solvent content at the terminal point    of the Step B of the web.

105. The method of item 104, wherein the residual poor solvent contentin the residual organic solvent content of the web is from 15 to 95% byweight at the terminal point of the Step B.

106. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction (Step B), the    temperature of the web at the beginning of the Step B being 30 to    130° C., the temperature of the web at the terminal point of the    Step B being 60 to 130° C., and the following relationship being    satisfied:    0.8×B0≦B1≦0.99×B0    wherein B0 (%) represents the residual solvent content at the    beginning of the Step B, and is 10 to 90% by weight of the film, and    B1 (%) represents the residual solvent content at the terminal point    of the Step B of the web.

107. The method of item 106, wherein the residual poor solvent contentin the residual solvent content of the web is from 15 to 95% by weightat the terminal point of the Step B.

108. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction (Step B), the    ambient temperature for the web in the step B being 110 to 140° C.,    and the following relationship being satisfied:    0.4×B2≦B3≦0.8×B2    wherein B2 (%) represents the residual solvent content at the    beginning of the Step B of the web, and B3 (%) represents the    residual solvent content at the terminal point of the Step B of the    web.

109. The method of item 108, wherein the residual poor solvent contentin the residual organic solvent content of the film is from 15 to 95% byweight at the terminal point of the Step B.

110. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction. (Step B), the    ambient temperature for the web in the step B being 30 to 130° C.,    and the following relationship being satisfied:    0.8×B0≦B1≦0.9×B0    wherein B0 (%) represents the residual solvent content at the    beginning of the Step B of the web, and B3 (%) represents the    residual solvent content at the terminal point of the Step B of the    web.

111. The method of item 110, wherein the residual poor solvent contentin the residual organic solvent content of the web is from 15 to 95% byweight at the terminal point of the Step B.

112. The method of any one of items 101 through 111, wherein the step Bis carried out at a stretching speed of from 50%/min to 500%/min, thestretching speed in the transverse direction being represented by thefollowing formula (3):Stretching speed (%/min)={(web length in the transverse direction afterstretching/web length in the transverse direction beforestretching)−1}×100/time (min) necessary to stretch.   formula (3)

113. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   holding the edges in the transverse direction of the peeled web    (Step A);-   stretching the cellulose ester film in the transverse direction    (Step B); and-   reducing a stretching tension (Step C),-   wherein the good solvent vapor concentration of the ambient air in    the steps A and B is from 2000 ppm to less than amount providing the    saturated vapor pressure.

114. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   stretching the peeled web in the transverse direction (Step B),    wherein the stretching magnification in the step B is from 1.1 to    2.5.

115. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   stretching the peeled web in the transverse direction (Step B);-   reducing a stretching tension of the web (Step C); and-   drying the film (Step D1) under the condition of DB<DD1, wherein DB    represents modulus of elasticity in the longitudinal direction of    the web at the terminal point of the Step B, and DD1 represents    modulus of elasticity in the longitudinal direction of the web at    the beginning of the Step D1.

116. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   stretching the peeled web in the transverse direction (Step B),    wherein the edges of the web are cut off with a slitter before the    Step B.

117. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web;-   holding the edges in the transverse direction of the web (Step A);-   stretching the held web in the transverse direction (Step B); and-   reducing a stretching tension of the web (Step C),-   wherein a neutral zone is provided between steps A and B or between    the steps B and C.

118. The method of any one of items 1 through 16, manufacturing acellulose ester film, the method comprising the steps of:

-   holding the edges in the transverse direction of the web (Step A);-   stretching the web in the transverse direction (Step B); and-   reducing a stretching tension of the web (Step C),-   wherein a neutral zone is provided between steps A and B or between    the steps B and C.

119. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a reducing a    stretching tension of the web (Step C);-   peeling the web;-   stretching the peeled web in the transverse direction,-   wherein Htd/Hmd is adjusted to be within the range of from 0.62 to    1.0 in which Htd and Hmd represent a tear strength in the transverse    direction of the film and a tear strength in the longitudinal    direction of the film, respectively.

120. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction,-   wherein Std is adjusted to be within the range of from −0.4 to 4.0%    and Smd to be within the range of from −0.4 to 4.0%, in which Std    and Smd represent a rate of dimensional variation in the transverse    direction of the film between the films before and after having been    heat treated at 80° C. and 90% RH for 100 hours and a rate of    dimensional variation in the longitudinal direction of the film    between the films before and after having been heat treated at    80° C. and 90% RH for 100 hours, respectively.

121. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the cellulose ester film; and-   stretching the peeled web in the transverse direction,-   wherein G2/G1 is adjusted to be within the range of from 0.9 to 1.0,    in which G1 and G2 represent the plasticizer content of the dope at    the casting step, and the plasticizer content of the film at the    terminal point of the Step D1, respectively.

122. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction,-   wherein the thickness dispersion R is adjusted to be within the    range of from 0 to 8%, R being represented by formula (4):    R(%)=(Rmax−Rmin)×100/Rave,  formula (4)    wherein Rmax, Rmin, Rave represent the maximum thickness in the    transverse direction of the film, the minimum thickness in the    transverse direction of the film, and the average thickness in the    transverse direction of the film, respectively.

123. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction,-   wherein the dispersion of the orientation angle is adjusted to be in    the direction inclined at an angle of 90°±1° to the longitudinal    direction of the film.

124. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction,-   wherein the dispersion of the retardation in the plane (R₀) of the    film is adjusted to be not more than 5%.

125. A method of manufacturing a cellulose ester film, the celluloseester film satisfying the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ (nm) represents a retardation in the plane of the film, andR_(t) (nm) represents a retardation in the thickness direction of thefilm; the method comprising the steps of:

-   casting a cellulose ester dope on a support to form a web;-   peeling the web; and-   stretching the peeled web in the transverse direction,-   wherein the dispersion of the retardation in the thickness direction    (R_(t)) of the film is adjusted to be not more than 10%.

126. The method of any one of items 101 through 125, wherein the haze ofthe film is adjusted to be in the range of from 0 to 2%.

127. The method of any one of items 101 through 126, wherein the totalacyl substitution degree of the cellulose ester film is from 2.3 to2.85, and the acetyl substitution degree of the cellulose ester film isfrom 1.4 to 2.85.

128. A cellulose ester film manufactured according to the method of anyone of items 1 through 27.

129. An optical retardation film comprising the cellulose ester film ofitem 128.

130. An optical compensation sheet comprising the cellulose ester filmof item 28 and provided thereon, an optically anisotropic layer.

131. An optical compensation sheet comprising the cellulose ester filmof item 128 and an optically anisotropic layer provided on the filmsurface which has contacted the support at the casting step.

132. An elliptic polarizing plate comprising the cellulose ester film ofitem 128.

133. An elliptic polarizing apparatus comprising the elliptic polarizingplate of item 132.

134. A cellulose ester film manufactured according to a methodcomprising the steps of casting a cellulose ester dope on a support toform a web and peeling the web, wherein Htd/Hmd is within the range offrom 0.62 to 1.0 in which Htd and Hmd represent a tear strength in thetransverse direction of the film and a tear strength in the longitudinaldirection of the film, respectively.

135. The cellulose ester film of item 134, wherein the cellulose esterfilm satisfies the following formula:Nx>Ny>Nzwherein Nx represents the refractive index in the transverse directionin the plane of the film, Ny represents the refractive index in thelongitudinal direction in the plane of the film, and Nz represents therefractive index in the thickness direction of the film.

136. The cellulose ester film of item 134 or 135, wherein the celluloseester film satisfies the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ represents a retardation in the plane of the film, and R_(t)represents a retardation in the thickness direction of the film.

137. The cellulose ester film of any one of items 134 through 136,wherein the total acyl substitution degree of the cellulose ester filmis from 2.3 to 2.85, and the acetyl substitution degree of the celluloseester film is from 1.4 to 2.85.

138. The cellulose ester film of any one of items 134 through 137,wherein the dispersion of the orientation angle is adjusted to be in thedirection inclined at an angle of 90°±1° to the longitudinal directionof thermoplastic particles film.

139. The cellulose ester film of any one of items 134 through 138,satisfying the following relationship:0(%)≦(Rmax−Rmin)×100/Rave≦8(%),wherein Rmax, Rmin, and Rave represent the maximum thickness in thetransverse direction of the film, the minimum thickness in thetransverse direction of the film, and the average thickness in thetransverse direction of the film, respectively.

140. The cellulose ester film of any one of items 134 through 139,wherein the dispersion of the retardation in the plane (R₀) of the filmis not more than 5%.

141. The cellulose ester film of any one of items 134 through 140,wherein the dispersion of the retardation in the thickness direction(R_(t)) of the film is be not more than 10%.

142. The cellulose ester film manufactured according to a methodcomprising the steps of casting a cellulose ester dope on a support toform a web and peeling the web, wherein Std is within the range of from−0.4 to 4.0% and Smd within the range of from −0.4 to 4.0%, in which Stdand Smd represent a rate of dimensional variation in the transversedirection of the film between the films before and after having beenheat treated at 80° C. and 90% RH for 100 hours and a rate ofdimensional variation in the longitudinal direction of the film betweenthe films before and after having been heat treated at 80° C. and 90% RHfor 100 hours, respectively.

143. The cellulose ester film of item 142, wherein the cellulose esterfilm satisfies the following formula:Nx>Ny>Nzwherein Nx represents the refractive index in the transverse directionin the plane of the film, Ny represents the refractive index in thelongitudinal direction in the plane of the film, and Nz represents therefractive index in the thickness direction of the film.

144. The cellulose ester film of item 142 or 143, wherein the celluloseester film satisfies the following formula:0.8≦R _(t) /R ₀≦3.5wherein R₀ represents a retardation in the plane of the film, and R_(t)represents a retardation in the thickness direction of the film.

145. The cellulose ester film of any one of items 142 through 144,wherein the total acyl substitution degree of the cellulose ester filmis from 2.3 to 2.85, and the acetyl substitution degree of the celluloseester film is from 1.4 to 2.85.

146. The cellulose ester film of any one of items 142 through 145,wherein the dispersion of the orientation angle is in the directioninclined at an angle of 90°±1° to the longitudinal direction of thefilm.

147. An optical retardation film comprising the cellulose ester film ofany one of items 134 through 146.

148. An optical compensation sheet comprising the cellulose ester filmof any one of items 134 through 146 and provided thereon, an opticallyanisotropic layer.

149. An elliptic polarizing plate comprising the cellulose ester film ofany one of items 134 through 146.

150. A displaying device comprising the elliptic polarizing plate ofitem 149.

A solution casting film forming process relating to a method formanufacturing a cellulose ester film will be explained below.

<<Solution Casting Film Forming Process>>

-   a) Dissolution step: The dissolution step is one in which cellulose    ester (in the flake form, powdery form or granular form, or in the    particles having an average particle size of preferably not less    than 100 μm), or additives are dissolved, while stirring, in organic    solvents mainly comprised of good solvents for the cellulose ester,    employing a dissolution vessel, and thereby a dope is prepared. In    order to carry out said dissolution, there are various methods such    as a method in which dissolution is carried out at a normal    atmospheric pressure, a method in which dissolution is carried out    at a temperature lower than the boiling point of the primary    solvent, a method in which dissolution is carried out at a    temperature higher than the boiling point of the main solvent under    an increase of pressure, a cooling dissolution method, a method in    which dissolution is carried out at a high pressure, and the like.    The resultant dope is filtered employing filter materials, is then    defoamed, and is subsequently pumped to the next process.

The dope is a solution in which the cellulose ester and additivesdescribed later are dissolved in an organic solvent.

(Cellulose Ester)

Raw materials for the cellulose ester used in the invention are notspecifically limited, and include cotton lint, tree pulp and kenaf. Thecellulose ester derived from these raw materials may be used incombination in an arbitrary amount ratio.

The cellulose ester in the invention is prepared by esterifyingcellulose as raw material with an acylating agent, for example, an acidanhydride (acetic anhydride, propionic anhydride, or butyric anhydride),in an organic acid such as acetic acid or in an organic solvent such asmethylene chloride in the presence of a protic catalyst such as sulfuricacid or by esterifying cellulose as raw material with an acylatingagent, an acid chloride (for example, CH₃COCl, C₂H₅COCl, or C₃H₇COCl) inthe presence of a basic compound as a catalyst such as an amine. Acellulose ester can be prepared according to a method described, forexample, in Japanese Patent O.P.I. Publication No. 10-45804. Thecellulose ester is obtained by substituting the hydrogen of the hydroxylgroup of the cellulose with an acyl group. The cellulose ester moleculeis comprised of many glucose units connected, each glucose unit havingthree hydroxyl groups. The number of hydrogen groups of the hydroxylgroup substituted by the acyl group is referred to as an acylsubstitution degree. For example, cellulose triacetate is one in whichall the hydrogens of the hydroxyl groups of cellulose are substitutedwith an acyl group.

As the cellulose ester in the invention are preferably used a celluloseester having in addition to an acetyl group, further an propionyl groupand/or a butyryl group such as cellulose acetate propionate, celluloseacetate butyrate (n-butyrate or iso-butyrate), or cellulose acetatepropionate butyrate (n-butyrate or iso-butyrate). A cellulose acetatepropionate having a high substitution degree of a propionate group hasexcellent water resistance.

The cellulose ester which can be used in the cellulose ester film of theinvention is not specifically limited, but is preferably a celluloseester satisfying the following formulae (I) and (II):2.3≦X+Y≦2.85  (I)1.4≦X≦2.85  (II)wherein X represents an acyl substitution degree and Y represents apropionyl substitution degree and/or a butyryl substitution degree.

The cellulose ester satisfying the above two formula is suitable for themanufacture of the cellulose ester film having an excellent opticalproperty for attaining the objects of the invention and excellent inheat resistance, which provides an optical retardation film having apositive wavelength dispersion and a good retardation. 2.5≦X+Y≦2.8 ismore preferred, and 2.6≦X+Y≦2.75 is still more preferred, in that acellulose ester film having a uniform optical property, particularlyreduced unevenness of retardation dispersion is obtained.

The acyl substitution degree of the cellulose ester can be measuredaccording to a method as defined in ASTM-817-96.

The number average molecular weight of the cellulose ester used in theinvention is preferably 60,000 to 300,000 in that a cellulose ester filmprepared therefrom has a high mechanical strength. The number averagemolecular weight of the cellulose ester used in the invention is morepreferably 70,000 to 200,000.

The number average molecular weight of the cellulose ester can bemeasured according to the following method.

-   Solvent used: Acetone-   Column used: MPW×1 (produced by Toso Co., Ltd.)-   Concentration of sample: 0.2 weight/volume %-   Flow rate: 1.0 ml/minute-   Injection amount of sample: 300 μl-   Standard sample: Polymethyl methacrylate (weight average molecular    weight Mw=188,200)-   Temperature: 23° C.    (Organic Solvent)

As an organic solvent, in which a cellulose ester is dissolved toprepare a cellulose ester dope, there is a chlorinated organic solventor a non-chlorinated organic solvent. Examples of the chlorinatedorganic solvent include methylene chloride, which is useful fordissolution of cellulose ester, particularly cellulose triacetate. Useof the non-chlorinated organic solvent is studied in view ofenvironmental problems. Examples of the non-chlorinated organic solventinclude methyl acetate, ethyl acetate, amyl acetate, acetone,tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, cyclohexanone, ethylformate, 2,2,2-trifluoroethanol, 2,2,3,3-tetrafluoro-1-propanol,1,3-difluoro-2-propanol, 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol,1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,3,3-pentafluoro-1-propanol, andnitroethane. As a dissolution method of a cellulose triacetate in thesesolvents, there is a method in which dissolution is carried out atordinary temperature. However a high temperature dissolution method, acooling dissolution method, a high pressure dissolution method ispreferably used in that undissolved matter can be minimized. As asolvent for cellulose esters other cellulose triacetate, methylenechloride can be used, and methyl acetate, ethyl acetate or acetone ispreferably used, and methyl acetate is more preferably used. In theinvention, an organic solvent capable of dissolving the cellulose estersdescribed above is referred to as a good solvent, and an organic solventused in a large amount to dissolve the cellulose esters is referred toas a main organic solvent.

The dope used in the in the invention preferably contains an alcoholhaving 1 to 4 carbon atoms in an amount of not less than 1 to 40% byweight. When a dope employing such an alcohol is cast on a metalsupport, and the solvent is evaporated to form a web (a dope film), theresidual alcohol content of the web increases during solventevaporation, and the residual alcohol as a gelling agent results ingelation of the web, whereby the web formed are easily peeled from thesupport. An organic solvent containing such an alcohol in a small amountincreases solubility of cellulose ester in an organic solvent containingno chlorine atom. The alcohols having 1 to 4 carbon atoms includemethanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, andtert-butanol.

Of these alcohol solvents, alcohol is preferred, which is less harmful,provides good dope stability, and has a relatively low boiling point andgood drying property. These alcohol solvents alone cannot dissolvecellulose ester and therefore belong to poor solvents. In the invention,a solvent other than the good solvent described above is defined as apoor solvent.

In the invention, the good solvent refers to a solvent capable ofdissolving not less than 5 g of the cellulose ester in 100 g of thesolvent at 25° C., and the poor solvent refers to a solvent incapable ofdissolving not less than 5 g of the cellulose ester in 100 g of thesolvent at 25° C.

In the invention, the concentration of the cellulose ester in the dopeis preferably 15 to 40% by weight, and the viscosity of the dope ispreferably 10 to 50 Pa·s, in view of quality of the cellulose ester filmsurface.

The dope used in the invention may contain various materials asdescribed below.

<Additives>

Additives such as a plasyicizer, a UV absorber, an antioxidizing agent,a dye or a matting agent can be added to the dope. These additives maybe mixed with cellulose ester or solvents in preparing a cellulose esterdope, or may be added to the cellulose ester dope while or after thedope is prepared. The cellulose ester film for a liquid crystal displaypreferably contains a plasticizer, an anti-oxidizing agent or a UVabsorber in that resistance to heat and humidity is secured.

<<Plasticizer>>

The dope used in the invention preferably contains a compound known asso-called “a plasticizer” in order to adjust mechanical properties,flexibility, a water absorption property, a moisture vapor transmittancerate, and a retardation of the cellulose ester film. For example, aphosphoric ester or a carboxylic acid ester is preferably used. Further,there are also preferably used a polymer prepared by polymerizing anethylenically unsaturated monomer having a weight average molecularweight of from 500 to 10,000, an acrylic polymer, and an acrylic polymerhaving on the side chain an aromatic ring or a cyclohexyl ring asdisclosed in Japanese Patent Application No. 2001-198450.

Examples of the phosphoric ester include triphenyl phosphate, tricresylphosphate, and phenyldiphenyl phosphate, trioctyl phosphate, andtributyl phosphate. Examples of the carboxylic acid ester includephthalate and citrate. Examples of the phthalate include dimethylphthalate, diethyl phthalate, dioctyl phthalate, and diethylhexylphthalate. Examples of the citrate include triethyl acetylcitrate andtributyl acetylcitrate. Examples of other carboxylic acid esters includebutyl oleate, metylacetyl ricinolate, dibutyl sebacate, and triacetin.Alkylphthalylalkyl glycolate is also preferably used. The alkyl of thealkylphthalylalkyl glycolate has a carbon atom number of preferably from1 to 8. Examples of the alkylphthalylalkyl glycolate includemethylphthalylmethyl glycolate, ethylphthalylethyl glycolate,propylphthalylpropyl glycolate, butylphthalylbutyl glycolate,octylphthalyloctyl glycolate, methylphthalylethyl glycolate,ethylphthalylmethyl glycolate, propylphthalylethyl glycolate,methylphthalylpropyl glycolate, methylphthalybutyl glycolate,ethylphthalybutyl glycolate, butylphthalymethyl glycolate,butylphthalyethyl glycolate, propylphthalybutyl glycolate,butylphthalypropyl glycolate, methylphthalyoctyl glycolate,ethylphthalyoctyl glycolate, octylphthalymethyl glycolate, andoctylphthalylethyl glycolate. Of these alkylphthalylalkyl glycolates,methylphthalylmethyl glycolate, ethylphthalylethyl glycolate,propylphthalylpropyl glycolate, butylphthalylbutyl glycolate, andoctylphthalyloctyl glycolate are preferable, and ethylphthalylethylglycolate is more preferable.

The plasticizer content of the cellulose ester film is preferably 1 to20% by weight based on the cellulose ester film, in realizing theeffects of the invention and in preventing the plasticizer from bleedingout from the cellulose ester film. Temperature is elevated to around200° C. while stretching or drying a cellulose ester web in themanufacture of the cellulose ester film. As a plasticizer used, aplasticizer having a vapor pressure at 200° C. of not more than 1333 Pais preferred in minimizing bleeding out of the plasticizer from thecellulose ester film.

<<Ultraviolet Absorber>>

The UV absorbers used in the invention include an oxybenzophenonecompound, a benzotriazole compound, a salicylic acid ester compound, abenzophenone compound, a cyanoacrylate compound and a nickel complexcompound. The benzotriazole compound with minimized undesired colorationis preferred. The UV absorbers disclosed in Japanese Patent O.P.I.Publication Nos. 10-182621 and 8-337574 or the polymeric UV absorbersdisclosed in Japanese Patent O.P.I. Publication No. 6-148430 are alsopreferably used.

The UV absorber in the invention is preferably a UV absorber which hasan excellent absorption of ultraviolet light having a wavelength of 370nm or less, in minimizing deterioration of a polarizing plate or aliquid crystal, and has a reduced absorption of visible light having awavelength of 400 nm or more, in view of image displaying.

Examples of the benzotriazole compound include2-(2′-hydroxy-5′-methylphenyl)-benzotriazole,2-(2′-hydroxy-3′,5′-di-tert-butylphenyl)-benzotriazole,2-(2′-hydroxy-3′-tert-butyl-5′-methylphenyl)-benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole,2-(2′-hydroxy-3′-(3″,4″,5″,6″-tetrahydrophthalimidomethyl)-5′-methylphenyl)-benzotriazole,2,2-methylenebis(4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazole-2-yl)phenol),2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole, and amixture of 2-(2H-benzotriazole-2-yl)-6-(straight-chained or brancheddodecyl)-4-methylphenol,octyl-3-[3-t-butyl-4-hydroxy-5-(chloro-2H-benzotriazole-2-yl)phenyl]propionateand2-ethylhexyl-3-[3-tert-butyl-4-hydroxy-5-(5-chloro-2H-benzotriazole-2-yl)phenyl]propionate,but are not limited thereto. TINUVIN 109, TINUVIN 171, and TINUVIN 326(each produced by Ciba Specialty Co., Ltd.), which are commerciallyavailable, are preferably used.

Examples of the benzophenone compound include 2,4-dihydroxybenzophenone,2,2′-dihydroxy-4-methoxybenzophenone,2-hydroxy-4-methoxy-5-sulfobenzophenone, and bis(2-methoxy-4-hydroxy-5-benzoylphenylmethane), but are not limitedthereto.

Of the above UV absorbers preferably used in the invention, thebenzotriazole or benzophenone UV absorber is preferably used which hashigh transparency, and minimizes deterioration of a polarizing plate ora liquid crystal. The benzotriazole UV absorber is especially preferablyused which minimizes undesired coloration. In the invention, the UVabsorber is dissolved in any solvent which can dissolve the UV absorber,and the UV absorber is preferably added to a cellulose ester solution inthe form of a UV absorber solution in which the UV absorber is dissolvedin a good solvent such as methylene chloride, methyl acetate ordioxolane or in the form of a UV absorber solution in which the UVabsorber is dissolved in a mixture solvent of a good solvent and a poorsolvent such as a lower alcohol (for example, methanol, ethanol,propanol or butanol) to obtain a dope. It is preferred that the solventcomposition of the UV absorber solution is the possible closest to thatof the dope. The UV absorber content of the dope is preferably 0.01 to5% by weight, and more preferably 0.5 to 3% by weight.

<<Anti-Oxidizing Agent>>

Hindered phenol compounds are preferably used as an anti-oxidizingagent. Examples of the hindered phenol compounds include2,6-di-t-butyl-p-cresol,pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine,2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, andtris-(3,5-di-t-butyl-4-hydroxybenzyl)-isocyanurate. Of these,2,6-di-t-butyl-p-cresol,pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],and triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] areespecially preferable. A metal-inactivating hydrazine compound such asN,N′-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionyl]-hydrazine or aphosphor-containing-stabilizer such as (2,4-di-t-butylphenyl)phosphitecan be used in combination. The content of these compounds in thecellulose ester film is preferably 1 ppm to 1.0% by weight, and morepreferably 10 to 1000 ppm by weight based on the cellulose ester weight.

<<Matting Agent>>

In the invention, a cellulose ester film containing a matting agent canprovide a good transportability or an easily windable property. As amatting agent, particles with the smallest possible particle size arepreferable. The matting agent particles include cross-linked polymerparticles and inorganic fine particles such as silicon dioxide, titaniumdioxide, aluminum oxide, zirconium oxide, calcium carbonate, kaolin,talc, burned calcium silicate, hydrated calcium silicate, aluminumsilicate, magnesium silicate, and calcium phosphate. Of these mattingagents, silicon dioxide is preferable in providing a reduced haze of thecellulose ester film.

The particles such as silicon dioxide particles are often surfacetreated with an organic compound, and such surface treated particles arepreferable in giving a reduced haze to the film. Examples of the organiccompound used in the surface treatment include halogenated silanes,alkoxysilanes, silazanes, and siloxanes.

The particles having a larger average particle size have a high slidingproperty, and on the contrary, the particles having a smaller averageparticle size have a good transparency. The secondary particles of theparticles have an average particle size of preferably 0.005 to 1.0 μm.The primary particles of the particles have an average particle size ofpreferably from 5 to 50 nm, and more preferably 7 to 14 nm. Since theseparticles contained in the cellulose ester film produce protrusions of0.01 to 1.0 μm on the surface of the cellulose ester film, they arepreferably used.

The particle content of the cellulose ester film is preferably from0.005 to 0.5% by weight, and more preferably from 0.05 to 0.4% byweight, based on the cellulose ester film.

Examples of the silicon dioxide particles include, for example, Aerosil200, 200V, 300, R972, R972V, R974, R202, R812, OX50, or TT 600 (eachproduced by Nihon Aerosil Co., Ltd.), and are preferably Aerosil 200V,R972, R972V, R974, R202 or R812. These particles may be used as amixture of two or more kinds thereof. When two or more kinds of theparticles are used, they may be mixed at any amount ratio. Two mattingagents different in kinds or average particle size, for example, Aerosil200V and R972 can be used in a 200V to R972 amount ratio of from0.1:99.9 to 99.9:0.01.

When the protrusions produced by the matting agent particles inhibitorientation of an orientation layer or a liquid crystal layer, which isprovided on a cellulose ester film, the cellulose ester film can beprepared so that the particles are located on only one side of the film.Coating a coating solution containing the matting agent and celluloseester (for example, diacetyl cellulose, cellulose acetate propionate,etc.) on the cellulose ester film surface reduces a coefficient offriction of the cellulose ester film, and improves slidability of thecellulose ester film.

<<Other Additives>>

The cellulose ester film may contain thermal stabilizers such asparticles of inorganic compounds including kaolin, kieselguhr, quartz,calcium carbonate, barium sulfate, titanium oxide and alumina, and saltsof alkali earth metals such as calcium and magnesium. Further, ananti-static agent, a flame retarder, a sliding agent, or oil may beadded to the cellulose ester film.

-   (b) Casting step: The casting step is one in which a dope is    conveyed to a pressure die through a pressure type metering gear    pump, and cast from said pressure die onto a casting site of a    moving endless metal belt such as a stainless steel belt or a metal    support such as a rotating metal drum. The surface of the metal    support for casting is specular. There is a doctor blade method in    which the thickness of the cast dope layer is controlled employing a    doctor blade or a reverse roll coater method in which the thickness    of the cast dope layer is controlled employing a roll rotating    reversely. The pressure die is preferred in which the slit shape at    the mouth piece portion can be regulated and the layer thickness is    readily controlled to be uniform. Examples of the pressure die    include a coat hanger die, a “T” die, and the like, and any of these    is preferably employed. In order to increase the casting speed, two    or more pressure dies may be provided on the metal support and dopes    divided into two or more may be simultaneously cast on the metal    support.-   (c) Solvent evaporation step: The solvent evaporation step is one in    which a web (refers to a dope layer formed after a dope is cast on a    metal support) is heated on a metal support and solvents are    evaporated till the web is capable of being peeled from the metal    support. In order to evaporate solvents, methods include a method in    which air is blown from the web side, and/or a method in which    heating is carried out from the reverse surface of the support    employing liquid, and a method in which heating is carried out from    the surface as well as the revere surface employing heat radiation.    Of these, the reverse surface liquid heating method is preferred due    to high drying efficiency. Further, these methods are preferably    combined.

In order to increase the film forming speed, a method is effective inwhich the temperature of the web on the metal support is elevated.However, the preferable drying speed is limited due to the compositionof the web, since excessive heating produces forms in the web whichresult from the solvents contained in the web. In order to increase thefilm forming speed, a method is preferred in which a dope is cast on ametal belt support. When the dope is cast on a metal belt support, thecasting speed is increased by lengthening the belt support. However, theincrease of the belt support length tends to cause deflection due to theweight of the belt support. This deflection causes vibration of thesupport, resulting in ununiformity of the web thickness at the casting.Therefore, the length of the belt support is preferably 40 to 120 m.

-   (d) Peeling step: The peeling step is one in which a web, which has    been subjected to evaporation of solvents on the support, is peeled    at the peeling site. The peeled web is conveyed to the subsequent    step. When the residual solvent amount (refer to the formula    described below) is too excessive, it may be difficult to peel the    web. On the contrary, when peeling is carried out after fully drying    the web on the support, a part of the web may peel prior to the    peeling site.

Listed as a method to increase the film forming speed is a gel castingmethod (in which peeling can be carried out even though the amount ofresidual solvents is relatively great). The gel casting methods includea method in which poor solvents with respect to the cellulose ester areadded to a dope and gelling is carried out after casting the dope, andalso a method in which gelling is carried out by decreasing thetemperature of a metal support, and the like. By strengthening the webthrough gelling the dope on the metal support, it is possible to carryout earlier peeling and to increase the casting speed. The web on themetal support can be peeled at the time when the residual solvent amountis in the range of 5 to 150% by weight depending on the dryingconditions or the length of the metal support.

It is preferred in the invention that at the peeling site on the metalsupport, temperature is 10 to 40° C., and preferably 15 to 35° C. It ispreferred in the invention that at the peeling site on the metalsupport, the residual solvent content of the web is 5 to 120% by weight.In the invention, the residual solvent content is expressed employingthe formula (1) described above.

When the web is formed on the belt support, increase in the film formingspeed increases amplitude of vibration of the belt support. Consideringthe residual solvent content of the web at peeling or the length of thebelt, the film forming speed is preferably 10 to 120 m/min, and morepreferably 15 to 60 m/min.

In the invention, the residual solvent content through the entire widthof the web is defined as an average residual solvent content. Theresidual solvent content occasionally refers to that at the limited webportions such as the center and the edges of the web.

-   (e) Drying step: The peeled web is generally dried at a drying step    of drying the web employing a drying apparatus in which said web is    alternatively passed through staggered rollers and/or employing a    tenter apparatus in which the web is conveyed while holding both    edges of the web employing clips. A common drying method is one in    which both surfaces of the web are heated by heated air flow.    Instead of the air flow, employed is a method in which heating is    carried out employing microwaves. Too rapid drying tends to degrade    the flatness of the finished film. During the entire drying step,    drying temperature is commonly from 30 to 250° C. Drying    temperature, drying time, and air volume for drying vary depending    on employed solvents. Thus, drying conditions may be properly    selected depending on types of employed solvents and their    combination.

In the invention, the step D0 refers to a step of transporting the web,which has been cast on a support and peeled from the support, to atenter section. Temperature in the step D0 is preferably controlled inorder to adjust a residual solvent content of the web at stretching. Thetemperature in the step D0, although it varies due to the residualsolvent content of the web in the step D0, is preferably 20 to 70° C.,more preferably 20 to 68° C., and most preferably 20 to 40° C., inminimizing stretching of the web in the transporting direction(hereinafter referred to as the longitudinal direction) and in adjustinga residual solvent content of the web at stretching.

In the step D0, ambient temperature distribution in the direction(hereinafter referred to as the transverse direction) perpendicular tothe transporting direction in the plane of the film is preferably narrowin that uniformity of the film is increased. The ambient temperaturedistribution in the step D0 is within the range of preferably ±5° C.,more preferably ±2° C., and most preferably ±1° C. of a predeterminedtemperature.

As a transporting tension of the web in the step D0, the preferred rangeas described below exists in determining the peeling condition of thecast film from the support or in minimizing stretching in thetransporting direction of the web in the step D0.

(Transporting Tension of the Web in the Step D0)

The transporting tension of the web in the step D0, although it dependson physical properties of the dope, the residual solvent content of thefilm at peeling or in the step D0, or temperature in the step D0, ispreferably 30 to 300 N/m, more preferably 57 to 284 N/m, and mostpreferably 57 to 170 N/m.

A tension cutting roller is preferably provided in order to minimizeelongation in the transporting direction of the web in the step D0.

The content ratio of the poor solvent to the good solvent in the web inthe step D0 is preferably limited to a preferred range in order tominimize elongation in the transporting direction of the web. The poorsolvent content (%) of the web at the terminal point of the step D0,which is represented by weight of poor solvent in the residual solventof the web at the terminal point of the step D0×100 (%)/(weight of poorsolvent in the residual solvent of the web at the terminal point of thestep D0+weight of good solvent in the residual solvent of the web at theterminal point of the step D0), is in the range of preferably 15 to 95%by weight, more preferably 25 to 95% by weight, and most preferably 30to 95% by weight.

-   (f) Stretching step (referred to also as tenter step)

Stretching step (referred to also as tenter step) in the invention willbe explained below employing FIG. 2.

In FIG. 2, a step A is a step of holding the edges in the transversedirection of the peeled web transported through the step D0 (notillustrated) of transporting the peeled web, a step B which follows is astep of stretching the web in the transverse direction (the directionperpendicular to the transporting direction of the web) preferably at astretching angle as shown in FIG. 1 described later, and a step C whichfurther follows is a step of reducing a tension of the stretched filmafter completion of the stretching.

It is preferred that a slitter for cutting off the web edges in thetransverse direction is provided between the film peeling step and thebeginning of the step B, and/or immediately after the step C. It isespecially preferred that the slitter is provided immediately before thebeginning of the step A. When stretching is carried out in thetransverse direction, a film, in which the film edges have been cut offbefore the beginning of the step B, provides more improved orientationangle dispersion as compared with a film in which the film edges are notcut off before the beginning of the step B.

This is considered to be because unintended stretching in thelongitudinal direction is minimized between the peeling step at whichthe film has a relatively high residual solvent and the step B.

It is also preferred that temperature sections, which have differentambient temperatures, are positively provided at the tenter step inorder to improve the orientation angle dispersion. Further, it is alsopreferred that neutral zones are provided between the two nearesttemperature sections so that the sections do not interfere with eachother. (Residual solvent content at the beginning of stretching in thestep B)

In the cellulose ester film manufacturing method of item 1 above, it isessential to manufacture a cellulose ester film so as to satisfy0.8≦R_(t)/R₀≦3.5, in which R₀ represents a retardation in the plane ofthe film, and R_(t) represents a retardation in the thickness directionof the film. In order to obtain the above range of R_(t)/R₀, theresidual solvent content of the web at the beginning of the step B is 5to 90% by weight, preferably 10 to 40% by weight, and more preferably 10to 35% by weight.

In the cellulose ester film manufacturing method of item 3 or 5 above,it is essential to manufacture a cellulose ester film so as to satisfy0.8≦R_(t)/R₀≦3.5, in which R₀ represents a retardation in the plane ofthe film, and R_(t) represents a retardation in the thickness directionof the film. In order to obtain the above range of R_(t)/R₀, theresidual solvent content of the web at the beginning of the step B ispreferably 10 to 90% by weight, more preferably 10 to 40% by weight, andmost preferably 10 to 35% by weight.

Further, in order to obtain the intended range of R_(t)/R₀ at the tenterstep, a preferred good solvent ambient concentration relationship existsamong the steps A, B and C. With respect to the preferred concentrationrelationship, Ma>Mc or Mb>Mc is preferred, wherein Ma, Mb, and Mcrepresent the good solvent ambient concentrations in the steps A, B, andC, respectively.

It is well known that the dispersion of an optical delayed phase axis(herein referred to also as the dispersion of an orientation angle) inthe transverse direction of the film is worsened at the step ofstretching the web in the direction perpendicular to the transportingdirection. In order to stretch the web so as to obtain a goodrelationship between R_(t) and R₀ and a good dispersion of theorientation angle, a preferred good solvent ambient concentrationrelationship exists among the steps A, B and C. When the good solventambient concentrations in the steps A, B, and C are Ma, Mb, and Mc,respectively, Ma is preferably more than 2000 ppm, more preferably morethan 3000 ppm, and most preferably more than 60% of the good solventsaturated vapor pressure concentration, Mb is preferably more than 2000ppm, more preferably more than 3000 ppm, and most preferably more than60% of the good solvent saturated vapor pressure concentration, or Mc ispreferably less than 60% of the good solvent saturated vapor pressureconcentration, more preferably less than 3000 ppm, and most preferablyless than more than 2000 ppm.

In order to stretch the web in the transverse direction so as to obtaina good relationship between R_(t) and R₀ and a good dispersion of theorientation angle, a preferred range of modulus of elasticity of the webexists in the steps A, B and C. When the moduli of elasticity of the webin the steps A, B, and C are Da, Db, and Dc, respectively, preferably100 N/mm²<|Da−Db|<2000 N/mm², more preferably 100 N/mm²<|Da−Db|<1000N/mm², and most preferably 100 N/mm²<|Da−Db|<700 N/mm².

Further, in order to obtain an intended range of R_(t)/R₀ at the tenterstep, a preferred range of modulus of elasticity of the web exists inthe steps A, B and C. When the moduli of elasticity of the web in thesteps A, B, and C are Da, Db, and Dc, respectively, preferably 500N/mm²<Db<2000 N/mm², more preferably 500 N/mm²<Db<1500 N/mm², and mostpreferably 500 N/mm²<Db<1000 N/mm².

(Content (%) of the Poor Solvent in the Residual Solvent of the Web)

In order to obtain an intended range of R_(t)/R₀ at the tenter step, apreferred range of the content of the poor solvent or the good solventin the residual solvent of the web exists at the terminal point of eachof the steps A, B and C. The poor solvent content (%) in the residualsolvent, which is represented by weight of poor solvent in the residualsolvent of the web×100 (%)/(weight of poor solvent in the residualsolvent of the web+weight of good solvent in the residual solvent of theweb), is in the range of preferably 15 to 95% by weight, more preferably25 to 95% by weight, and most preferably 30 to 95% by weight at each ofthe terminal points of the steps A, B and C. Further, the above poorsolvent content may be the same or different at each of the terminalpoints of the steps A, B and C.

(Temperature and Residual Solvent Content of the Web at the Steps A, Band C)

It is well known that the dispersion of an optical delayed phase axis(herein referred to also as the dispersion of an orientation angle) inthe transverse direction of the film is worsened at a step of stretchingthe web in the transverse direction. In order to stretch the web so asto obtain a good relationship between R_(t) and R₀ and a good dispersionof the orientation angle, a preferred web temperature relationshipexists among the steps A, B and C. When the web temperatures at theterminal points of the steps A, B, and C are Ta (° C.), Tb (° C.), andTc (° C.), respectively, Ta≦Tb−10 or Tc≦Tb is preferable, and Ta≦Tb−10and Tc≦Tb are more preferable.

When the web is stretched in the direction perpendicular to thetransporting direction at the tenter step, in order to reduce thedispersion of the orientation angle and obtain a good range of R_(t)/R₀at the tenter step, it is preferred that the web in a relatively softstate is stretched in the step B, and the web in the steps A and C isharder as compared with that in the step B. The above conditions can berealized controlling web temperature or the residual solvent content ofthe web. Ambient temperature at each step depends upon the residualsolvent content of the web, but it is preferred that temperature in thestep A is 30 to 40° C., and temperature in the steps B and C is 30 to140° C. When the residual solvent content of the web at the terminalpoint of the step B is 0.4 to 0.8 times that of the film at thebeginning of the step B, ambient temperature of the step B is preferablyfrom 110 to 140° C. When the residual solvent content of the web at theterminal point of the step B is 0.4 to 0.8 times that of the web at thebeginning of the step B, it is also preferred that ambient temperatureat the beginning of the step B is from 30 to 140° C., and ambienttemperature at the terminal point of the step B is from 70 to 140° C.When the residual solvent content of the web at the terminal point ofthe step B is 0.8 to 0.99 times that of the web at the beginning of thestep B, ambient temperature of the step B is preferably from 30 to 130°C. When the residual solvent content of the web at the terminal point ofthe step B is 0.8 to 0.99 times that of the web at the beginning of thestep B, it is also preferred that ambient temperature at the beginningof the step B is from 30 to 130° C., and ambient temperature at theterminal point of the step B is from 60 to 130° C.

In order to obtain a good dispersion of an orientation angle and apreferable range of R_(t)/R₀, the temperature elevation rate of the webin the step B is preferably 0.5 to 10° C./s.

In order to obtain a preferable range of R_(t)/R₀, stretching in thestep B is preferably carried out at a short time. However, the range oftime necessary to stretch the web is limited in view of uniformity ofthe film. The range is preferably 1 to 10 seconds, and more preferably 4to 10 seconds.

The heat conduction coefficient at the tenter step may be constant orvaried. The heat conduction coefficient is in the range of preferablyfrom 41.9 to 419×10³ J, more preferably from 41.9 to 209.5×10³ J, andmost preferably from 41.9 to 126×10³ J.

In order to obtain an intended range of R_(t) and R₀, the stretchingrate of the web in the transverse direction in the step B may beconstant or varied. The stretching rate is preferably 50 to 500%/min,more preferably 100 to 400%/min, and most preferably 200 to 300%/min.

At the tenter step, the ambient temperature distribution of the web hasa preferred range, since its narrow distribution is preferable inincreasing uniformity of the film. The ambient temperature distributionat the tenter step is within the range of preferably ±5° C., morepreferably ±2° C., and most preferably ±1° C. of a predeterminedtemperature. Such a narrow temperature distribution as described aboveis expected to provide a narrow temperature distribution in thetransverse direction of the web.

(Stretching Angle in the Preparation of Cellulose Ester Film)

The web cast on a support is stretched in the transverse direction inthe step B of the cellulose ester film manufacturing method of theinvention. Herein, one embodiment of the stretching step will beexplained below employing FIG. 1.

The stretching angle in the step B as shown in FIG. 1 is preferably 2 to10°, more preferably 3 to 7°, and most preferably 3 to 5°.

In order to obtain a preferred retardation in the invention, thestretching magnification at stretching the web in the transversedirection is preferably 1.1 to 2.5, more preferably 1.3 to 1.8, and mostpreferably 1.3 to 1.6.

When in the invention the web is stretched in the step B, a uniaxialstretching apparatus or a biaxial stretching apparatus-may be used.

In the step C, a tension in the direction perpendicular to the webtransporting direction is preferably reduced in order to minimize adimensional variation of the film. The tension is reduced so that thewidth of the web is preferably 95 to 99.5% of the web width in theprevious step.

A drying step (hereinafter referred to also as step D1) is preferablyprovided after the tenter step. In order to further refine opticalproperties given to the cellulose ester film in the tenter step, and drythe web, the web is preferably subjected to heat treatment preferably at50 to 140° C., more preferably at 80 to 140° C., and most preferably at80 to 130° C.

In order to further refine optical properties given to the celluloseester film in the tenter step and dry the web, the web is preferablysubjected to heat treatment at a coefficient of heat transfer ofpreferably from 20.9 to 126×10³ J/m²hr, more preferably from 41.9 to126×10³ J/m²hr, and most preferably from 41.9 to 83.7×10³ J/m²hr.

(Temperature Distribution at Step D1)

In the step D1, ambient temperature in the plane and in the directionperpendicular to the transporting direction of the web is preferablynarrow in that uniformity of the film is increased. The ambienttemperature distribution in the step D1 is within the range ofpreferably ±5° C., more preferably ±2° C., and most preferably ±1° C. ofa predetermined temperature.

(Transporting Tension of the Web in the Step D1)

As a transporting tension of the web in the step D1, a preferred rangeexists in minimizing stretching in the transporting direction of theweb. The transporting tension of the web in the step D1, although itdepends on physical properties of the dope, the residual solvent contentof the film at peeling or in the step D1, or temperature in the step D1,is preferably 120 to 200 N/m, more preferably 140 to 200 N/m, and mostpreferably 140 to 160 N/m.

A tension cut roller is preferably provided in order to minimizeelongation in the transporting direction of the web in the step D1.After drying, the edges in the transverse direction of the web arepreferably cut off through a slitter before uptaken, in obtaining a goodroll formation.

(Shear Strength in the Transverse Direction and in the LongitudinalDirection of the Cellulose Ester Film)

When the web is stretched in the transverse direction, the web ispreferably stretched so that the ratio of shear strength in thetransporting direction (hereinafter referred to also as MD) of thecellulose ester film and shear strength in the direction (hereinafterreferred to also as TD) perpendicular to the transporting direction ofthe cellulose ester film falls within a certain range. When shearstrength in the TD and shear strength in the MD are Htd and Hmd,respectively, the ratio Htd/Hmd satisfies preferably the firstrelationship 0.6<Htd/Hmd<1, more preferably the second relationship0.783<Htd/Hmd<1, and most preferably the third relationship0.83<Htd/Hmd<1.

(Rate of Dimensional Variation in the Transverse Direction and in theLongitudinal Direction of the Cellulose Ester Film)

When the web is stretched in the transverse direction, the web ispreferably stretched so that rate of dimensional variation falls withina certain range. When rate of dimensional variation in the TD of thecellulose ester film and rate of dimensional variation in the MD of thecellulose ester film are Std and Smd, respectively, it is preferred thatStd is less than 0.4% and Smd is more than −0.4%, it is more preferredthat Std is less than 0.25% and Smd is more than −0.25%, and it is stillmore preferred that Std is less than 0.2% and Smd is more than −0.2%.

When the web is stretched in the transverse direction, the web ispreferably stretched so that a rate of variation of the plasticizer orUV absorber content of the web at the completion of stretching fallswithin a certain range.

The rate of variation of the plasticizer or UV absorber content of theweb herein referred to is represented by the following:(Plasticizer or UV Absorber Content of the Cellulose Esterdope−plasticizer or UV absorber content of the web at the completion ofstretching)×100 (%)/Plasticizer or UV absorber content of the celluloseester dope

The rate of variation of the plasticizer content of the web ispreferably not more than 10%, more preferably not more than 7%, and mostpreferably not more than 5%. The rate of variation of the UV absorbercontent of the web is preferably not more than 10%, more preferably notmore than 7%, and most preferably not more than 5%.

It is also preferred that G2/G1 falls within the range of from 0.9 to1.0, in which G1 and G2 represent the plasticizer content of the dope atthe casting step, and the plasticizer content of the web at the terminalpoint of the Step D1, respectively.

When the web is stretched in the transverse direction, the web ispreferably stretched so that degree of crystallinity of the web afterstretched falls within a certain range. When the web is stretched in thetransverse direction, the web is preferably stretched so thatconcentration distribution of the plasticizer in the web after stretchedfalls within a certain range.

Next, the cellulose ester film manufactured according to the method asdescribed above will be explained.

(Retardation (R₀, R_(t)) of Cellulose Ester Film)

In order to obtain a cellulose ester film which is employed in theoptical retardation film, optical compensation sheet or ellipticpolarizing plate of the invention, and which provides a liquid crystaldisplay with a wide viewing angle, the web is stretched in thetransverse direction in the manufacture of the cellulose ester film sothat the ratio R_(t)/R₀ of the cellulose ester film satisfies thefollowing formula:0.8≦R _(t) /R ₀≦3.5

The ratio R_(t)/R₀ is preferably 0.8 to 3.0, and more preferably 0.8 to2.5.

The retardation in the plane (R₀) of the cellulose ester film of theinvention is preferably 30 to 1000 nm, more preferably 30 to 500 nm,still more preferably 30 to 150 nm, and most preferably 30 to 75 nm. Theretardation in the thickness direction (R_(t)) of the cellulose esterfilm of the invention is preferably 30 to 1000 nm, more preferably 30 to500 nm, and still more preferably 30 to 250 nm.

When the web is stretched in the transverse direction, the web ispreferably stretched so that the orientation angle dispersion in thetransverse direction of the film falls within a certain range. In theorientation angle of any point measured in the transverse direction ofthe film, the deviation from the average orientation angle is within therange of preferably ±2%, more preferably ±0.1%, and still morepreferably ±0.5%.

(Orientation Angle)

In the invention, the orientation angle represents an angle between thedelayed phase axis direction in the film plane and the transverse orlongitudinal direction in the manufacturing process of the celluloseester film according to a cast film forming method. The orientationangle is measured employing an automatic birefringence meterKOBURA-A21ADH.

As one of the reasons that disorder of the orientation angle of thecellulose ester film manufactured by stretching the web in thetransverse direction occurs, it is considered that unintended stretchingin the longitudinal direction of the web occurs. Where the unintendedstretching in the longitudinal direction of the web occurs, the film hasa retardation in the plane having a delayed phase axis in thelongitudinal direction of the film and increases a retardation in thethickness direction of the film, resulting in deterioration ofdispersion of the orientation angle. In order to provide a retardationin the plane having a delayed phase axis in the transverse direction ofthe film, the retardation in the plane having a delayed phase axis inthe longitudinal direction of the film is requires to be cancelled,which results in an increases of a retardation in the thicknessdirection of the film. Accordingly, uniformalization of the orientationangle uniformalizes retardations both in the plane and in the thicknessdirection, which provides a low R_(t)/R₀.

(Dispersion of Retardation of Cellulose Ester Film)

In the invention, in the dispersion of the retardation in the plane (R₀)of the cellulose ester film, the cellulose ester film has a coefficientof variation of (R₀) of preferably not more than 5%, more preferably notmore than 2%, and still more preferably not more than 1.5%. In thedispersion of the retardation in the thickness direction (R_(t)) of thecellulose ester film, the cellulose ester film has a coefficient ofvariation of (R_(t)) of preferably not more than 10%, more preferablynot more than 2%, and still more preferably not more than 1.5%.

The value of the dispersion of the retardation as described above isrepresented in terms of a coefficient of variation of retardationsobtained by being measured at an interval of 1 cm in the transversedirection of the film. The method of measuring the retardation and thevalue of the dispersion will be described later.

(Variation of Retardation Due to Different Wavelengths of Light forMeasurement)

In the cellulose ester film prepared by stretching the web in thetransverse direction, a smaller variation of retardation due to thedifferent wavelengths provides a liquid crystal display panel withminimized color irregularity.

The ratios R₄₅₀/R₀ and R₆₅₀/R₀ satisfy preferably the first relationship0.5<R₄₅₀/R₀<1.0 and 1.0<R₆₅₀/R₀<1.5, more preferably the secondrelationships 0.7<R₄₅₀/R₀<0.95 and 1.01<R₆₅₀/R₀<1.2, and still morepreferably the third relationships 0.8<R₄₅₀/R₀<0.93 and1.02<R₆₅₀/R₀<1.1.

(Light Transmittance of Cellulose Ester Film)

As a film for an LCD (liquid crystal display), high light transmittanceand high ultraviolet ray absorption are required. The cellulose esterfilm, to which the additives described above are added, has a lighttransmittance at 500 nm of preferably from 85 to 100%, more preferablyfrom 90 to 100%, and still more preferably from 92 to 100%. The film hasa light transmittance at 400 nm of preferably from 45 to 100%, morepreferably from 50 to 100%, and still more preferably from 60 to 100%.Further, the film has a light transmittance at 380 nm of preferably from0 to 10%, more preferably from 0 to 5%, and still more preferably from 0to 3%.

(Dispersion of Thickness in the Transverse Direction of Cellulose EsterFilm)

The web is stretched in the transverse direction so that the dispersionR(%) of thickness in the transverse direction of the film satisfiespreferably the first relationship 0≦R≦8, more preferably the secondrelationship 0≦R≦5, and still more preferably the third relationship 0≦R≦4.

(Haze of Cellulose Ester Film)

As one of the reasons that an increase in haze of the cellulose esterfilm prepared by stretching the web in the transverse direction occurs,it is considered that unintended stretching in the longitudinaldirection of the web occurs. Stretching the web so that haze of the filmis reduced uniformalizes a retardation in the plane and a retardation inthe thickness direction, providing a low R_(t)/R₀.

It is preferred that the cellulose ester film is stretched in thetransverse direction so that the film after stretched has a haze fallingwithin a certain range. The haze of the film is preferably not more than2%, more preferably not more than 1.5%, and still more preferably notmore than 1%.

(Coefficient of Elasticity of Cellulose Ester Film)

It is preferred that the web is stretched in the transverse direction sothat the film has a tensile strength falling within a certain range.

It is preferred that the web is stretched in the transverse direction sothat the film has a modulus of elasticity falling within a certainrange. Modulus of elasticity in the transverse direction (TD) andmodulus of elasticity in the longitudinal direction (MD) may be the sameor different. In the web stretched in the transverse direction,unintended stretching in the transverse direction of the web results inchange of modulus of elasticity of the film. Stretching the web so thatthe film has a modulus of elasticity falling within a certain rangeuniformalizes retardation in the plane and retardation in the thicknessdirection, which provides a low R_(t)/R₀.

The modulus of elasticity of the cellulose ester film is in the range ofpreferably from 1.5 to 5 GPa, more preferably from 1.8 to 4 GPa, andstill more preferably from 1.9 to 3 GPa.

In the web stretched in the transverse direction, unintended stretchingin the transverse direction results in change of fracture point stressof the film. Stretching the web so that the film has a fracture pointstress falling within a certain range uniformalizes a retardation in theplane and a retardation in the thickness direction, which provides a lowR_(t)/R₀. The fracture point stress in the transverse direction (TD) andthe fracture point stress in the longitudinal direction (MD) may be thesame or different.

The fracture point stress of the cellulose ester film is in the range ofpreferably from 50 to 200 MPa, more preferably from 70 to 150 MPa, andstill more preferably from 80 to 100 MPa.

In the web stretched in the transverse direction, unintended stretchingin the transverse direction results in change of fracture pointelongation of the film. Stretching the web so that the film has afracture point elongation falling within a certain range uniformalizes aretardation in the plane and a retardation in the thickness direction,which provides a low R_(t)/R₀. Fracture point elongation in thetransverse direction (TD) and fracture point elongation in thelongitudinal direction (MD) may be the same or different.

The fracture point elongation of the cellulose ester film is in therange of preferably from 20 to 80%, more preferably from 30 to 60%, andstill more preferably from 40 to 50%.

In the web stretched in the transverse direction, unintended stretchingin the transverse direction results in change of rate of hygroscopicswelling of the film. It is preferred that the web is stretched in thetransverse direction so that the film has a rate of hygroscopic swellingfalling within a certain range. Rate of hygroscopic swelling in thetransverse direction (TD) and rate of hygroscopic swelling in thelongitudinal direction (MD) may be the same or different.

The rate of hygroscopic swelling of the cellulose ester film is in therange of preferably from −1 to 1%, more preferably from −0.5 to 0.5%,and still more preferably from 0 to 0.2%.

In the web stretched in the transverse direction, unintended stretchingin the transverse direction tends to produce luminescent foreignmaterials. Stretching the web so that occurrence of the luminescentforeign materials is minimized uniformalizes a retardation in the planeand a retardation in the thickness direction, which provides a lowR_(t)/R₀.

The luminescent foreign materials on the cellulose ester film is in therange of preferably from 0 to 80 per cm² of the film, more preferablyfrom 0 to 60 per cm² of the film, and still more preferably from 0 to 30per cm² of the film.

Generally, when a cellulose ester film is used as a polarizing plateprotective film, the film is subjected to alkali saponification in orderto improve its adhesion to a polarizing film. After the cellulose esterfilm is alkali saponified, the resulting film is adhered to thepolarizing film through a polyvinyl alcohol aqueous solution as anadhesive. Therefore, when a contact angle of the saponified celluloseester film to water is high, the adhesion through the polyvinyl alcoholaqueous solution is difficult, which is problematic. The contact angleof the saponified cellulose ester film to water is in the range ofpreferably from 0 to 60°, more preferably from 5 to 55°, and still morepreferably from 10 to 30°.

(Center-Line Average Roughness Ra of Cellulose Ester Film)

When a cellulose ester film is used as a member for an LCD, the film isrequired to have an excellent flatness, in order to minimize leak oflight. Herein, the center-line average roughness Ra is a value definedin JIS B 0161. The measurement is carried out, for example, according toa trace method or an optical method. The center-line average roughness(Ra) of the cellulose ester film in the invention is preferably not morethan 20 nm, more preferably not more than 10 nm, and still morepreferably not more than 3 nm.

Next, the measuring method of various measurements with respect to thecellulose ester film in the invention and the measuring method used inexamples described later will be explained.

(Poor Solvent Content in the Residual Solvent)

The residual solvent of the cellulose ester film sample was collectedunder reduced pressure, and solvents in the collected solvent werequantitatively analyzed according to gas chromatography.

(Modulus of Elasticity, Fracture Point Elongation, and Fracture PointStress)

These were measured according to tensile testing as follows. Thecellulose ester film containing a residual solvent was cut to a size ofa width of 10 mm and a length of 200 mm, and fixed by two chucks 100 mmdistant from each other at a given temperature in a methylene chloridesaturated atmosphere, and then tensile testing was carried out at atensile speed of 100 mm/min.

The cellulose ester film containing no residual solvent was cut to asize of a width of 10 mm and a length of 200 mm, and fixed by two chucks100 mm distant from each other at a given temperature, and then tensiletesting was carried out at a tensile speed of 100 mm/min.

Measurement of retardations R₀ and R_(e), refractive index in thedelayed phase axis direction (in the transverse direction) Nx,refractive index in the advanced phase axis direction (in thelongitudinal direction) Ny, refractive index in the thickness directionNz, and the delayed phase axis direction (orientation angle)

Measurement of the three dimensional birefringence of the celluloseester film was made at 23° C. and 55% RH employing light with at awavelength of 590 nm by means of an automatic birefringence meterKOBRA-21ADH (produced by Oji Keisokukiki Co., Ltd.), then theretardations R₀ and R_(e) were determined, and refractive indices Nx, Nyand Nz, and the delayed phase axis direction were determined. Theorientation angle is an angle between the delayed phase axis directionin the film plane and the transverse or longitudinal direction of thefilm.

(Dispersion of Retardations (R₀ and R_(t)) and a Coefficient ofVariation (CV) thereof)

Measurement of the three dimensional birefringence of the celluloseester film was made at an interval of 1 cm in the transverse directionat 23° C. and 55% RH employing light with at a wavelength of 590 nm bymeans of an automatic birefringence meter KOBRA-21ADH (produced by OjiKeisokukiki Co., Ltd.) to obtain retardations in the plane andretardations in the thickness direction. The standard deviation of theresulting retardations was obtained according to a (n−1) method. In theexperiments, “n” was 130 to 140. Regarding the dispersion of theretardation, a coefficient of variation (CV) of the retardation isdetermined which is represented by the following formula:Coefficient of variation (CV) of the retardation=Standarddeviation/Average of retardations(Wavelength Dispersion Property)

Refractive indices at wavelengths 450 nm and 650 nm in the three axisdirections of the cellulose ester film were measured at 23° C. and 55%RH by means of an automatic birefringence meter KOBRA-21ADH (produced byOji Keisokukiki Co., Ltd.) to obtain R₄₅₀ and R₆₅₀, respectively.

As the wavelength dispersion property in the invention, R₄₅₀/R₀ andR₆₅₀/R₀ were evaluated.

(Shear Strength of Dry Cellulose Ester Film)

The cellulose ester film sample was subjected to humidity conditioningat 23° C. and at 55% RH for 4 hours, and cut into a size of 50 mm×64 mm,and then the shear strength of the resulting sample was measuredaccording to ISO 6383/2-1983.

(Rate of Dimensional Variation)

The cellulose ester film sample was subjected to humidity conditioningat 23° C. and at 55% RH for 4 hours, and two points about 10 cm distantfrom each other were recorded with a mark on the sample surface in eachof the transverse direction and the longitudinal direction, employing aknife, and the distances L1 in both directions between the two pointswere measured. Thereafter, the sample was subjected to high temperatureand high humidity conditioning at 60° C. and at 90% RH for 24 hours, andthen again subjected to humidity conditioning at 23° C. and at 55% RHfor 4 hours, and then the distances L2 in both directions between thetwo points were measured. The rate of dimensional variation isrepresented by the following formula:Rate (%) of dimensional variation={(L2−L1)/L1}×100(Rate of Hygroscopic Swelling)

The cellulose ester film sample was subjected to humidity conditioningat 23° C. and at 55% RH for 4 hours, and two points about 20 cm distantfrom each other were recorded with a mark on the sample surface in eachof the transverse direction and the longitudinal direction, employing aknife, and the distances L3 in both directions between the two pointswere measured. Thereafter, the sample was subjected to high temperatureand high humidity conditioning at 60° C. and at 90% RH for 24 hours in athermostat, and then the distances L4 in both directions between the twopoints were measured in not more than 2 minutes after the sample wastaken out from the thermostat. The rate of hygroscopic swelling isrepresented by the following formula:Rate (%) of hygroscopic swelling={(L4−L3)/L4}×100(Dispersion of Thickness)

The cellulose ester film sample was subjected to humidity conditioningat 23° C. and at 55% RH for 4 hours, and the thickness of the sample wasmeasured at an interval of 10 mm in the transverse direction. Based onthe measurements, the dispersion of thickness R (%) represented by thefollowing formula was computed:Dispersion of thickness R(%)={R(max)−R(min)}×100/R(ave)wherein R(max) represents the maximum thickness, R(min) represents theminimum thickness, and R(ave) represents the average thickness.(Haze)

Haze of the cellulose ester film sample was measured according to JISK-6714 employing a haze meter TYPE 1001DP (produced by Nihon Denshokukogyo Co., Ltd., and was used as a measure of transparency.

(Measurement of Light Transmittance)

Spectral transmittances τ (λ) of the cellulose ester film sample weremeasured at an interval of 10 nm in the wavelength range of from 350 to700 nm, employing a spectral photometer U-3400 (produced by HitachiSeisakusho Co., Ltd). Light transmittance at 380 nm, 400 nm and 500 nmwas computed from the resulting spectral transmittances τ (λ).

(Curl Value)

The cellulose ester film sample was stored at 25° C. and 55% RH for 3days, and cut into a piece with a size of 2 mm (in the longitudinaldirection)×50 mm (in the transverse direction). The resulting film piecewas further subjected to humidity conditioning at 23±2° C. and 55% RHfor 24 hours. Thereafter, curl of the resulting piece was determinedemploying a curvature scale. The curl value was measured according to amethod A defined in JIS-K7619-1988.

The curl value was represented in terms of 1/R, in which R (m) is theradius of curvature of the circle and the unit of R is meter (m).

(Luminescent Foreign Materials)

A polarizing film was inserted between two cellulose ester film samplesarranged in a crossed Nicol state to prepare a polarizing plate toprepare a polarizing plate sample. The number of luminescent foreignmaterials on the polarizing plate sample was counted, employing amicroscope by a factor of 30. The luminescent foreign materials wereobserved as bright materials which when light is projected to one sideof the plate sample, appears on the other side of the plate sample. Thenumber per 250 mm² of luminescent foreign materials at ten areas of thepolarizing plate sample were counted, and the number per cm² ofluminescent foreign materials was determined. In the invention,luminescent foreign materials observed had a size of from 5 to 50 μm²,and no luminescent foreign materials having a size exceeding 50 μm² wereobserved.

(Contact Angle of Saponified Cellulose Ester Film)

The cellulose ester film sample was treated with a 2.5N NaOH solution at50° C. for 2.5 minutes, washed with pure water for 2.5 minutes, andsubjected to humidity conditioning at 23° C. and 55% RH for 24 hours.Contact angle of the resulting sample to water was measured through acontact angle meter TYPE CA-D, which was produced by Kyowa Kaimen KagakuCo., Ltd.

(Center Line Average Roughness Ra)

The center line average roughness Ra of the cellulose ester film samplewas measured through a non-contact surface roughness meter WYKO NT-2000.

(Image Sharpness)

The image sharpness is defined according to JIS K-7105. The imagesharpness measured at a 1 mm slit is preferably not less than 90%, morepreferably not less than 95%, and still more preferably not less than99%.

(Rate of Water Absorption)

The cellulose ester film sample was cut into a 10×10 cm² film sample,immersed in 23° C. water for 24 hours, and taken out from the water.Water on the surface of the sample was softly wiped with a filter paper.The resulting sample had a weight of W1. Subsequently, the sample wassubjected to humidity conditioning at 23° C. and at 55% RH for 24 hours,and the resulting sample had a weight of W0. The rate of waterabsorption was obtained from the following formula:Rate of water absorption (%)=(W1−W0)×100/W0(Rate of Water Content)

The cellulose ester film sample was cut into a 10×10 cm² film sample,subjected to humidity conditioning at 23° C. and at 80% RH for 48 hours,and the resulting sample had a weight of W3. Subsequently, the samplewas dried at 120° C. for 45 minutes, and the resulting sample had aweight of W2. The rate of water content at 23° C. and at 80% RH wasobtained from the following formula:Rate of water content (%)={(W3−W2)/W2}×100(Moisture Vapor Transmittance)

Moisture vapor transmittance herein referred to is a value according toa method described in JIS Z 0208. The moisture vapor transmittance ofthe cellulose ester film of the invention is preferably 10 to 250g/m²·24 hours, more preferably 20 to 200 g/m²·24 hours, and still morepreferably 50 to 180 g/m²·24 hours.

The optical retardation film (called also optical retardation plate)will be explained below.

The cellulose ester film of the invention can be used as an opticalretardation film increasing a viewing angle of a liquid crystal display.

The optical compensation sheet of the invention will be explained below.

An optically anisotropic layer containing an optically anisotropiccompound is provided on the cellulose ester film of the invention or onanother layer provided on the cellulose ester film of the invention.Thus, the optical compensation sheet of the invention is obtained.

When an optically anisotropic layer containing an optically anisotropiccompound is provided on the cellulose ester film of the inventionstretched in the transverse direction, the anisotropic layer ispreferably provided on the surface of the cellulose ester film on theside contacting a support for casting the cellulose ester dope in themanufacture of the film, in view of high smoothness of the film surface.

When a compound having a liquid crystal property is coated on thecellulose ester film of the invention stretched in the transversedirection to form an optically anisotropic layer, the anisotropic layeris preferably provided on the surface of the cellulose ester filmopposite the side contacting a support for casting the cellulose esterdope in the manufacture of the film, in view of orientation of thecompound.

The polarizing plate of the invention and the liquid crystal display ofthe invention employing the polarizing plate will be explained below.

A conventional polarizing film can be used as the polarizing film usedin the polarizing plate of the invention. For example, a polarizingfilm, in which a film of a hydrophilic polymer such as polyvinyl alcoholis treated with a dichromatic dye such as iodine, and stretched, or apolarizing film, in which a plastic film such as polyvinyl chloride issubjected to orientation treatment, can be used. The thus obtainedpolarizing film is laminated with a cellulose ester film.

In the invention, one in which the optical compensation sheet of theinvention is provided on at least one side of the polarizing film asdescribed above is employed as the polarizing plate. When the opticalcompensation sheet is provided on only one side of the polarizing film,the cellulose ester film of the invention or other transparent supportsuch as TAC (cellulose triacetate) film is provided on the other side ofthe polarizing film.

The cellulose ester film of the invention is preferably employed as apolarizing plate protective film. For example, a liquid crystal iscoated on the cellulose ester film or on an orientation layer providedon the cellulose ester film, oriented and fixed to form an opticallyanisotropic layer. The cellulose ester film with the opticallyanisotropic layer is employed as the polarizing plate protective film,whereby a polarizing plate providing a viewing angle increasing effectcan be prepared.

When the web is stretched in the transverse direction, the web ispreferably stretched so that the surface energy of the web afterstretched falls within a certain range.

When the web is stretched in the transverse direction, the web ispreferably stretched so that the conductivity of the web after stretchedfalls within a certain range.

When the web is stretched in the transverse direction, the web ispreferably stretched so that the curl value of the web after stretchedfalls within a certain range.

The curl value is preferably −20 to 20 (1/m), more preferably −15 to 15(1/m), and still more preferably −10 to 10 (1/m) at 23° C. and 55% RH.

When the cellulose ester film is employed as a polarizing plateprotective film, the rate of water absorption of the cellulose esterfilm is preferably 1.0 to 4.5%, in that durability of the polarizingplate is improved and a drying property is improved when the polarizingplate protective film is adhered to a polarizing film to prepare apolarizing plate.

The rate of water content of the cellulose ester film is preferably 0.5to 4.5%, more preferably 1.5 to 3.5%, and still more preferably 1.5 to3.0%.

Thus obtained polarizing plate may be provided on one side of a liquidcrystal cell or on each side of a liquid crystal cell. When thepolarizing plate is provided on only one side of a liquid crystal cell,the polarizing plate is laminated on the liquid crystal cell so that theoptical compensation sheet is closer to the liquid crystal cell than thepolarizing film to form the liquid crystal display of the invention.

In a liquid crystal cell for driving of a liquid crystal display, apolarizing film is ordinarily provided on each of a first substrateprovided on one side of the liquid crystal cell and a second substrateprovided on the other side of the liquid crystal cell. At least oneoptical compensation sheet may be provided between the first substrateand the liquid crystal cell and/or between the second substrate and theliquid crystal cell. In the invention, a single optical compensationsheet of the invention is preferably provided between the substrateprovided on the liquid crystal cell for driving and the polarizing filmon a viewer side, in order to reduce cost and effectively realize theobject of the invention.

In a twisted nematic (TN) mode liquid crystal display, the opticalcompensation sheet of the invention is provided on a substrate of the TNmode liquid crystal cell so that the direction providing a maximumrefractive index in the plane of the cellulose ester film support of theoptical compensation sheet is substantially normal to the orientationdirection of the substrate (or the liquid crystal closest to thesubstrate) of the liquid crystal cell. The direction providing a maximumrefractive index in the plane of the cellulose ester film support isinclined at an angle of 90°±5°, and preferably 90° to the orientationdirection of the substrate of the liquid crystal cell.

EXAMPLES

The invention will be detailed according to the following examples, butis not limited thereto. The term, “part” and “parts” represent “part byweight” and “parts by weight”, respectively, unless otherwise specified.

Example 1

<<Preparation of Cellulose Ester Film 1>>

A dope and a UV absorber solution were prepared as described below, andcellulose ester film 1 was prepared employing the dope and UV absorbersolution.

(Preparation of Dope)

Cellulose acetate propionate (with an acetyl substitution degree of 2.00and a propionyl substitution degree of 0.80 and a viscosity averagepolymerization degree of 350) of 100 parts, 2 parts ofethylphthalylethyl glycolate, 8.5 parts of triphenyl phosphate, 290parts of ethylene chloride, and 60 parts of ethanol were placed into atightly sealed vessel, and gradually heated to 45° C. in 45 minuteswhile slowly stirred. Pressure in the vessel was 1.2 atmosphere. Theresulting solution was filtered employing Azumi filter paper No. 244,produced by Azumi Roshi Co., Ltd., and allowed to stand for 24 hours toremove foams in the solution. Thus, a cellulose ester solution wasprepared.

(Preparation of UV Absorber Solution)

Five parts of the cellulose acetate propionate described above, 6 partsof TINUVIN 326 (produced by Ciba Specialty Co., Ltd.), 4 parts ofTINUVIN 109 (produced by Ciba Specialty Co., Ltd.), and 5 parts ofTINUVIN 171 (produced by Ciba Specialty Co., Ltd.) were dissolved in asolvent of 94 parts of methylene chloride and 8 parts of ethanol. Thus,UV absorber solution was obtained.

One hundred parts of the cellulose ester solution were mixed with 2parts of the UV absorber solution and the mixture was stirred with astatic mixer to prepare a dope. The dope of 30° C. was cast at a widthof 1.6 m on a stainless steel belt support, dried for 1 minute on thestainless steel whose the rear side was brought into contact with 25° C.water, and maintained for 15 seconds on the stainless steel cooled whoserear side was further brought into contact with a 15° C. chilled waterto form a web. Then, the web was peeled the support.

The residual solvent content at the peeling of the web was 80% byweight. The peeled web was transported in a D0 step at a transportingtension of 100 N/m. Ethanol/(ethanol+methylene chloride) at the terminalpoint of the D0 step was 70% by weight.

Both edges of the peeled web were held with clips in the step A and thedistance between the clips was changed by applying a tension in thetransverse direction of the web at a stretching speed of 250%/min in thestep B, employing a uniaxial stretching tenter. The ambient temperaturewas 120° C. and the stretching magnification was 1.5.

At the beginning of the stretching, the web temperature was 80° C. andthe residual solvent content of the web was 25% by weight. At theterminal point of the stretching, the web temperature was 120° C., theresidual solvent content of the web was of 60% of that at the beginningof the stretching, and ethanol/(ethanol+methylene chloride) was 93% byweight.

Next, in the step C, the stretched web was transported, reducing thetension to provide 98% of the web width in the step B. The ambientconcentration of methylene chloride in the steps A and B was 4000 ppm.The ambient concentration of methylene chloride in the step C was notmore than 60% of the saturated concentration. Subsequently, theresulting web was dried at an ambient temperature of 100° C. in the stepD1. Thus, a cellulose ester film 1 was obtained.

At the terminal point of the step B, the coefficient of elasticity ofthe web was 600 N/mm², and at the beginning of the step D1, thecoefficient of elasticity of the web was 2000 N/mm².

The resulting cellulose ester film was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for a knurling treatment in order to prevent film adherencein the roll film.

A cellulose ester film sample was taken from the resulting film roll,and in the central portion of the sample, the refractive index Nx in thetransverse direction, the refractive index Ny in the longitudinaldirection, and the refractive index Nz in the thickness direction weremeasured. Nx, Ny, and Nz were 1.47783, 1.47703, and 1.47613,respectively. The thickness of the sample was 80 μm.

Further, the optical properties were computed.Nx−Ny=0.0008(Nx+Ny)/2−Nz=0.0013

-   R₀=64 nm-   R_(t)=104 nm    R _(t) /R ₀=1.63-   Dispersion of R₀=1.4%-   Dispersion of R_(t)=1.6%

The central portion of the sample had a haze of 0.1%.

As is apparent from the above, cellulose ester film 1 has excellentoptical properties as an optical retardation film, or an opticalcompensation sheet.

Example 2

<<Preparation of Cellulose Ester Film 2>>

In the same manner as in Example 1, the dope was cast on a stainlesssteel belt support to form a web, and the web was peeled. The residualsolvent content at the peeling of the web was 90% by weight. The peeledweb was transported in the D0 step at a transporting tension of 100 N/m.Ethanol/(ethanol+methylene chloride) at the terminal point of the D0step was 62% by weight.

Both edges of the peeled web were held with clips in the step A and thedistance between the clips was changed by applying a tension in thetransverse direction of the web at a stretching speed of 250%/min in thestep B, employing a uniaxial stretching tenter. The ambient temperaturewas 170° C. and the stretching magnification was 1.5.

At the beginning of the stretching, the web temperature was 80° C. andthe residual solvent content of the web was 30% by weight. At theterminal point of the stretching, the web temperature was 100° C., theresidual solvent content of the web was of 90% of that at the beginningof the stretching, and ethanol/(ethanol+methylene chloride) was 66% byweight.

Next, in the step C, the stretched web was transported, reducing thetension to provide 98% of the web width in the step B. The ambientconcentration of methylene chloride in the steps A and B was 4000 ppm.The ambient concentration of methylene chloride in the step C was notmore than 60% of the saturated concentration. Subsequently, theresulting web was dried at an ambient temperature of 100° C. in the stepD1. Thus, a cellulose ester film 2 was obtained.

At the terminal point of the step B, the coefficient of elasticity ofthe web was 500 N/mm², and at the beginning of the step D1, thecoefficient of elasticity of the web was 1800 N/mm².

The resulting cellulose ester film was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for a knurling treatment to form protrusions with a heightof about 10 μm in order to prevent film adherence in the roll film.

A cellulose ester film sample was taken from the resulting film roll,and in the central portion of the sample, the refractive index Nx in thetransverse direction, the refractive index Ny in the longitudinaldirection, and the refractive index Nz in the thickness direction weremeasured. Nx, Ny, and Nz were 1.47790, 1.47710, and 1.47600,respectively. The thickness of the sample was 80 μm.

Further, the optical properties were computed.Nx−Ny=0.0008(Nx+Ny)/2−Nz=0.0015

-   R₀=64 nm-   R_(t)=120 nm    R _(t)/R₀=1.88-   Dispersion of R₀=1.3%-   Dispersion of R_(t)=1.4%

The delayed phase axis direction of the sample was within the range of±0.4 degree to the transverse direction of the film. The central portionof the sample had a haze of 0.1%.

As is apparent from the above, cellulose ester film 2 has excellentoptical properties as an optical retardation film, or an opticalcompensation sheet.

Example 3

<<Preparation of Cellulose Ester Film 3 (Comparative)>>

In the same manner as in Example 1, the dope was cast on a stainlesssteel belt support to form a web, and the web was peeled. The residualsolvent content at the peeling of the web was 90% by weight. The peeledweb was transported in the D0 step at a transporting tension of 100 N/m.Ethanol/(ethanol+methylene chloride) at the terminal point of the D0step was 95% by weight.

Both edges of the peeled cellulose ester film were held with clips inthe step A and the distance between the clips was changed by applying atension in the transverse direction of the web at a stretching speed of250%/min in the step B, employing a uniaxial stretching tenter. Theambient temperature was 170° C. and the stretching magnification was1.5.

At the beginning of the stretching, the web temperature was 150° C. andthe residual solvent content of the web was 4% by weight. At theterminal point of the stretching, the web temperature was 170° C., theresidual solvent content of the web was of 30% of that at the beginningof the stretching, and ethanol/(ethanol+methylene chloride) was 98% byweight.

The ambient concentration of methylene chloride in the steps A and B was4000 ppm. Next, the stretched web was transported and then dried at anambient temperature of 100° C. in the step D1. Thus, comparativecellulose ester film 3 was obtained. In the step B, the coefficient ofelasticity of the web was 1500 N/mm², and in the step D1, thecoefficient of elasticity of the film was 2000 N/mm².

The resulting cellulose ester film was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for a knurling treatment in order to prevent film adherencein the roll film.

A cellulose ester film sample was taken from the resulting film roll,and in the central portion of the sample, the refractive index Nx in thetransverse direction, the refractive index Ny in the longitudinaldirection, and the refractive index Nz in the thickness direction weremeasured. Nx, Ny, and Nz were 1.47792, 1.47742, and 1.47567,respectively. The thickness of the sample was 80 μm. From these values,Nx−Ny, (Nx+Ny)/2−Nz, R₀, and R_(t) were computed as 0.0005, 0.002, 40.0nm, and 160.0 nm, respectively.

R_(t) is far higher than R₀, which is problematic for an opticalretardation film. Further, the delayed phase axis direction of thesample exceeds ±0.2 degree to the transverse direction of the film.

Further, the other optical properties of the comparative cellulose esterfilm sample 3 are collectively shown below.

Dispersion of a retardation in the plane R₀ 5.1% Dispersion of aretardation in the 11.2% thickness direction R_(t) Average thicknessR(ave) 80 μm Maximum thickness R(max) 83.4 μm Minimum thickness R(min)76.1 μm Dispersion R of thickness 9.1% Light transmittance (500 nm) 80%Light transmittance (400 nm) 78% Light transmittance (380 nm) 12% Haze2.1% Center line average roughness Ra 21.1 nm Rate of dimensionalvariation (Smd) −0.41% Rate of dimensional variation (Std) −0.51% Rateof hygroscopic swelling (MD) 1.2% Rate of hygroscopic swelling (TD) 2.3%Shear strength (MD) 2.2 N/μm Shear strength (TD) 1.2 N/μm Htd/Hmd 0.6Fracture point stress (MD) 45 MPa Fracture point stress (TD) 32 MPaFracture point elongation (MD) 15% Fracture point elongation (TD) 12%Modulus of elasticity (MD) 1.4 GPa Modulus of elasticity (TD) 1.3 GPaCurl (23° C., 55% RH) 45 m⁻¹ Luminescent foreign materials 85/cm²Plasticizer content ratio G2/G1 0.88 Contact angle of the saponifiedfilm 65°

Example 4

<<Preparation of Cellulose Ester Films 4 through 9>>

Cellulose ester films 4 through 9 were prepared in the same manner as inExample 1, except that cellulose acetate propionate with an acetylsubstitution degree of 1.8 and a propionyl substitution on degree of0.80 was used and the ratio Htd/Hmd, the ratio of shear strength in thetransverse direction of the film to shear strength in the longitudinaldirection of the film was adjusted as shown in Table 1.

A retardation in the plane R₀ and a retardation in the thicknessdirection R_(t) of the resulting film were determined, and the rationR_(t)/R_(o) were computed. The results are shown in Table 1.

TABLE 1 Cellulose ester film Htd/Hmd R_(t)/R₀ 4 0.59 3.8 5 0.62 2.0 60.78 2.0 7 0.83 1.2 8 1.0 1.5 9 1.1 3.6

As is apparent from Table 1 above, cellulose ester films 5 through 8having a tear strength ratio H_(td)/H_(md) of from 0.62 to 1.0 haveexcellent optical properties as an optical retardation film, or anoptical compensation sheet.

Example 5

<<Preparation of Cellulose Ester Films 10 through 18>>

Cellulose ester films 10 through 18 were prepared in the same manner asin Example 1, except that cellulose acetate propionate with an acetylsubstitution degree of 2.00 and a propionyl substitution degree of 0.7was used and the film were stretched in the transverse direction to givea rate of dimensional variation in the transverse direction Std (%) anda rate of dimensional variation in the longitudinal direction Smd (%) asshown in Table 2.

Further, the ratio R_(t)/R₀ of the resulting film was computed. Theresults are shown in Table 2.

TABLE 2 Cellulose ester film Smd (%) Std (%) R_(t)/R₀ 10 −0.01 −0.02 1.111 0.2 −0.01 1.5 12 0.2 0.15 1.6 13 −0.2 −0.25 1.5 14 −0.32 −0.28 1.9 15−0.27 −0.32 1.9 16 0.32 0.28 1.9 17 0.41 −0.20 3.1 18 −0.19 0.42 3.2

As in apparent from Table 2 above, cellulose ester films 11 through 16prepared by stretching so as to have an Smd falling within the range offrom −0.4 to 0.4% and an Std falling within the range of from −0.4 to0.4% have excellent optical properties as an optical retardation film oran optical compensation sheet.

Example 6

<<Preparation of Cellulose Ester Films 19 through 23>>

Cellulose ester films 19 through 23 were prepared in the same manner asin Example 1, except that cellulose acetate propionate with an acetylsubstitution degree of 1.9 and a propionyl substitution degree of 0.7was used and the film were stretched in the transverse direction to givea dispersion R of thickness in the transverse direction of the film asshown in Table 3.

Further, the ratio R_(t)/R₀ of the resulting film was computed. Theresults are shown in Table 3.

TABLE 3 Cellulose ester film R (%) R_(t)/R₀ 19 3.5 1.1 20 4.0 1.5 21 5.02.2 22 8.0 2.8 23 8.5 3.5

As is apparent from Table 3 above, cellulose ester films 19 through 23,prepared by stretching so as to have a dispersion R of thickness fallingwithin the range of from 0 to 8%, have excellent optical properties asan optical retardation film or an optical compensation sheet.

Example 7

<<Preparation of Polarizing Plate>>

(Preparation of Elliptic Polarizing Plate A for Viewing AngleCompensation)

A triacetyl cellulose film with a thickness of 80 μm (produced by KonicaCorporation) was immersed in an aqueous 2 mole/liter sodium hydroxidesolution at 60° C. for 2 minutes, and dried at 100° C. for 10 minutes.Thus, a triacetyl cellulose support subjected to alkali saponificationwas obtained.

A 120 μm thick polyvinyl alcohol film was immersed in an aqueoussolution comprised of 1 part by weight of iodine, 4 parts by weight ofboric acid and 100 parts by weight of water, and stretched at 50° C. bya factor of 4 to obtain a polarizing film (polarizer 1).

The triacetyl cellulose support obtained above was adhered, through a 5%completely saponified polyvinyl alcohol aqueous solution as an adhesive,to each side of the above obtained polarizer 1. Thus, polarizing plate 1was prepared.

<Preparation of Oriented film A-2>

A gelatin layer (having a thickness of 0.1 μm) was coated on one side ofthe above polarizer 1, and a solution, in which 1.5% of alkyl modifiedpolyvinyl alcohol having the following chemical structure was dissolvedin a mixture solvent of methanol/water (=1:3), was coated on the gelatinlayer, employing a wire bar #3, and dried at 65° C. employing hot air.Then the resulting alkyl modified polyvinyl alcohol layer was subjectedto a rubbing treatment, so that the rubbing direction was parallel withthe direction of the absorption axis of the polarizing plate, to form anorientation layer. Thus, polarizing plate 1 a was obtained.

A solution LC-1 described below was coated on the orientation layer ofthe polarizing plate 1 a above employing a wire bar #6, dried at 55° C.for 30 seconds, heated at 100° C. for 30 seconds, gradually cooled,nitrogen purged at 98 kPa for 60 seconds, and hardened with ultravioletlight of 450 mJ under an oxygen concentration of 0.1% to form a hardenedlayer. Thus, polarizing plate 1 b having one optically anisotropiclayer, in which orientation of the liquid crystal compounds was fixed,was obtained.

Next, the cellulose ester film 1 prepared in Example 1 was laminated onthe optically anisotropic layer of the polarizing plate 1 b obtainedabove through an adhesive SK Dain 2092 so that the direction of thetransmission axis of the polarizer was in accordance with that providinga maximum refractive index of the cellulose ester film 1. Thus, anelliptic polarizing plate A for viewing angle compensation was obtained.

(Composition of solution LC-1) MEK (methylethyl ketone) 86 partsCompound 2  3 parts Compound 3  2 parts Compound 4  3 parts Compound 5 3 parts Ilugacure 369 (produced by Ciba Specialty Chemicals Co., Ltd.) 1 part

<<Evaluation of Elliptic Polarizing Plate A for Viewing AngleCompensation)

The optical compensation sheet and polarizing plate of the liquidcrystal panel of a 15 TYPE liquid crystal display Multi Sync LCD1525J,produced by NEC Co., Ldt., were peeled from the liquid crystal cell, andthe elliptic polarizing plate A was laminated thereto so that theabsorption axis of the elliptic polarizing plate A was in accordancewith that of the polarizing plate before being peeled. Thus, a liquidcrystal panel sample was obtained.

The viewing angle of the resulting sample was measured employing anEZ-Contrast produced by ELDIM Co., Ltd. The viewing angle wasrepresented by a range of an angle inclined to the direction normal tothe plane of the liquid crystal panel showing a contrast ratio duringwhite/black display of 10 or more and showing image reversal.

The inventive samples exhibited a greatly increased viewing angle.

Example 8

<<Preparation of Cellulose Ester Film 24>>

In the same manner as in Example 1, the dope was cast on a stainlesssteel belt support to form a web, and the web was peeled. The residualsolvent content at the peeling of the web was 90% by weight. The peeledweb was transported in the DO step at a transporting tension of 150 N/m.Ethanol/(ethanol+methylene chloride) at the terminal point of the DOstep was 30% by weight.

Both edges of the peeled web were cut off with a slitter by a 10 cmwidth and held with clips in the step A, employing a uniaxial stretchingtenter.

Then, the distance between the clips was changed by applying a tensionin the transverse direction of the web at a stretching speed of 130%/minin the step B. The ambient temperature was 110° C. and the stretchingmagnification was 1.4. At the beginning of the stretching, the webtemperature was 60° C. and the residual solvent content of the web was20% by weight. At the terminal point of the stretching, the webtemperature was 100° C., the residual solvent content of the web was of90% of that at the beginning of the stretching, andethanol/(ethanol+methylene chloride) was 93% by weight.

Next, in the step C, the web was transported, reducing the tension so asto provide 97% of the web width in the step B. The ambient concentrationof methylene chloride in the steps A and B was 2000 ppm. The ambientconcentration of methylene chloride in the step C was not more than 60%of the saturated concentration.

A neutral zone was provided between the steps A and B and between thesteps B and C in order to prevent the temperatures of the differentsteps from interfering with each other.

Subsequently, the resulting web was dried at an ambient temperature of100° C. in the step D1. Thus, a cellulose ester film 24 was obtained.

At the terminal point of the step B, the coefficient of elasticity ofthe web was 550 N/mm², and at the beginning of the step D1, thecoefficient of elasticity of the web was 1800 N/mm².

The resulting cellulose ester film 24 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for a knurling treatment to form protrusions with a heightof about 10 μm in order to prevent film adherence in the roll film.

Optical properties of the cellulose ester film 24 are collectively shownbelow.

Nx 1.4778 Ny 1.4772 Nz 1.4760 R₀ 50 nm R_(t) 120 nm R_(t)/R₀ 2.4 R₄₅₀/R₀0.88 R₆₅₀/R₀ 1.03 Dispersion of orientation angle within the range of±0.5° Dispersion of a retardation R₀ 1% Dispersion of a retardationR_(t) 1.4% Average thickness R(ave) 80.0 μm Maximum thickness R(max)80.5 μm Minimum thickness R(min) 79.5 μm Dispersion R of thickness 1.3%Light transmittance (500 nm) 92.2% Light transmittance (400 nm) 58.2%Light transmittance (380 nm) 2.3% Haze 0.1% Center line averageroughness Ra 1.65 nm Rate of dimensional variation (Smd) −0.05% Rate ofdimensional variation (Std) −0.03% Rate of hygroscopic swelling (MD)0.1% Rate of hygroscopic swelling (TD) 0.07% Shear strength (MD) 2.3N/μm Shear strength (TD) 2.0 N/μm Fracture point stress (MD) 93 MPaFracture point stress (TD) 87 MPa Fracture point elongation (MD) 40%Fracture point elongation (TD) 45% Modulus of elasticity (MD) 2.8 GPaModulus of elasticity (TD) 3.0 GPa Curl (23° C., 55% RH) 8 m⁻¹Luminescent foreign materials 21/cm² Plasticizer content ratio G2/G10.99 Contact angle of the saponified film 17°

As is apparent from the above, cellulose ester film 24 has excellentoptical properties as an optical retardation film or an opticalcompensation sheet.

A viewing angle compensation elliptic polarizing plate was prepared inthe same manner as in Example 7, except that the cellulose ester film 24was used instead of the cellulose ester film 1, and was evaluated forviewing angle in the same manner as in Example 7. As a result, thepolarizing plate obtained above exhibited greatly improved viewing angleas compared with the optical compensation sheet and polarizing plateused in the liquid crystal panel of Multi Sync LCD 1525J.

Example 9

A polarizer 1 was prepared in the same manner as in Example 7. Thecellulose ester film 24 prepared in Example 8 was immersed in an aqueous2 mole/liter sodium hydroxide solution at 60° C. for 2 minutes, anddried at 100° C. for 10 minutes. Thus, a transparent support B subjectedto alkali saponification was obtained.

A gelatin layer (having a thickness of 0.1 μm) was coated on the surfaceof the transparent support B opposite the side contacting the beltsupport at the casting of the cellulose ester dope in the manufacture ofthe cellulose ester film. A solution, in which 1.5% of the alkylmodified polyvinyl alcohol described above was dissolved in a mixturesolvent of methanol/water (=1:3), was coated on the gelatin layer,employing a wire bar #3, and dried at 65° C. employing hot air. Then,the resulting alkyl modified polyvinyl alcohol coat layer was subjectedto a rubbing treatment to form an orientation layer. Herein, the rubbingtreatment was carried out so that the rubbing direction was parallelwith the longitudinal direction of the cellulose ester film.

The solution LC-1 described previously was coated on the orientationlayer above employing a wire bar #6, dried at 55° C. for 30 seconds,heated at 100° C. for 30 seconds, gradually cooled, nitrogen purged at98 kPa for 60 seconds, and hardened with ultraviolet light of 450 mJunder an oxygen concentration of 0.1% to form a hardened layer. Thus, anoptical compensation sheet C having an optically anisotropic layercontaining the liquid crystal compounds, in which orientation of theliquid crystal compounds was fixed, was obtained.

Next, the polarizer 1 obtained above was laminated on the opticallyanisotropic layer of the optical compensation sheet C through anadhesive SK Dain 2092 so that the direction of the transmission axis ofthe polarizer was in accordance with that of the delayed phase axis ofthe transparent support B. Thus, an elliptic polarizing plate D forviewing angle compensation was obtained.

The resulting viewing angle compensation elliptic polarizing plate D wasevaluated for viewing angle in the same manner as in Example 7. As aresult, the polarizing plate D obtained above exhibited a greatlyimproved viewing angle property.

Example 10

An elliptic polarizing plate E for viewing angle compensation-wasprepared in the same manner as in Example 9, except that a gelatin layer(having a thickness of 0.1 μm) was coated on the surface of thetransparent support B, the surface contacting the belt support atcasting of the cellulose ester dope in the manufacture of the celluloseester film. The resulting elliptic polarizing plate E for viewing anglecompensation was evaluated for viewing angle in the same manner as inExample 7. As a result, the polarizing plate E obtained above exhibiteda greatly improved viewing angle property.

Example 11

<<Preparation of Cellulose Ester Film 25>>

Cellulose ester film 25 was prepared in the same manner as in Example 8,except that cellulose acetate propionate with an acetyl substitutiondegree of 1.9 and a propionyl substitution degree of 0.75 was usedinstead of the cellulose acetate propionate used in Example 1.

Optical properties of the cellulose ester film 25 are collectively shownbelow.

Nx 1.4781 Ny 1.4772 Nz 1.4758 R₀ 70 nm R_(t) 150 nm R_(t)/R₀ 2.1 R₄₅₀/R₀0.91 R₆₅₀/R₀ 1.04 Dispersion of orientation angle within the range of±0.5° Dispersion of a retardation R₀ 1.2% Dispersion of a retardationR_(t) 1.5% Average thickness R(ave) 80.0 μm Maximum thickness R(max)80.6 μm Minimum thickness R(min) 79.4 μm Dispersion R of thickness 1.5%Light transmittance (500 nm) 92.1% Light transmittance (400 nm) 58.4%Light transmittance (380 nm) 2.3% Haze 0.1% Center line averageroughness Ra 1.66 nm Rate of dimensional variation (Smd) −0.05% Rate ofdimensional variation (Std) −0.08% Rate of hygroscopic swelling (MD)0.1% Rate of hygroscopic swelling (TD) 0.07% Shear strength (MD) 2.3N/μm Shear strength (TD) 2.0 N/μm Fracture point stress (MD) 91 MPaFracture point stress (TD) 87 MPa Fracture point elongation (MD) 39%Fracture point elongation (TD) 45% Modulus of elasticity (MD) 2.8 GPaModulus of elasticity (TD) 3.0 GPa Curl (23° C., 55% RH) 8 m⁻¹Luminescent foreign materials 23/cm² Plasticizer content ratio G2/G10.99 Contact angle of the saponified film 18°

As is apparent from the above, cellulose ester film 25 has excellentoptical properties as an optical retardation film or an opticalcompensation sheet.

EFFECTS OF THE INVENTION

The present invention can provide a manufacturing method of a celluloseester film having excellent optical properties, and the cellulose esterfilm manufactured according to the method, and further provide anoptical retardation film, an elliptic polarizing plate, an opticalcompensation sheet and an image display each employing the celluloseester film.

1. A cellulose ester film manufactured according to a method comprisingthe steps of: casting a cellulose ester dope on a support to form a web;and peeling the web, wherein the cellulose ester film satisfies thefollowing formula:Nx>Ny>Nz wherein Nx represents the refractive index in the transversedirection in the plane of the film, Ny represents the refractive indexin the longitudinal direction in the plane of the film, and Nzrepresents the refractive index in the thickness direction of the film,wherein Htd/Hmd is within the range of from 0.62 to 1.0 in which Htd andHmd represent a tear strength in the transverse direction of the filmand a tear strength in the longitudinal direction of the film,respectively, and wherein the dispersion of the orientation angle of thecellulose ester film fall within a direction inclined at an angle of90°±1° to the longitudinal direction of the film.
 2. The cellulose esterfilm of claim 1, wherein the cellulose ester film satisfies thefollowing formula:0.8≦R _(t) /R ₀≦3.5 wherein R₀ represents a retardation in the plane ofthe film, and R_(t) represents a retardation in the thickness directionof the film.
 3. The cellulose ester film of claim 1, wherein the totalacyl substitution degree of the cellulose ester film is from 2.3 to2.85, and the acetyl substitution degree of the cellulose ester film isfrom 1.4 to 2.35.
 4. The cellulose ester film of claim 1, furthersatisfying the following relationship:0(%)≦(Rmax−Rmin)×100/Rave≦8 (%), wherein Rmax, Rmin, and Rave representthe maximum thickness in the transverse direction of the film, theminimum thickness in the transverse direction of the film, and theaverage thickness in the transverse direction of the film, respectively.5. The cellulose ester film of claim 1, wherein the dispersion of theretardation in the plane (R₀) of the film is not more than 5%.
 6. Thecellulose ester film of claim 1, wherein the dispersion of theretardation in the thickness direction (R_(t)) of the film is not morethan 10%.
 7. The cellulose ester film of claim 1, wherein Std is withinthe range of from −0.4 to 0.4% and Smd is within the range of from −0.4to 0.4%, in which Std and Smd represent a rate of dimensional variationin the transverse direction of the film between the films before andafter having been heat treated at 80° C. and 90% RH for 100 hours and arate of dimensional variation in the longitudinal direction of the filmbetween the films before and after having been heat treated at 80° C.and 90% RH for 100 hours, respectively.
 8. An optical compensation sheetcomprising the cellulose ester film of claim 1, and provided thereon, anoptically anisotropic layer.
 9. A polarizing plate comprising thecellulose ester film of claim 1, and a polarizing film in which apolyvinyl alcohol film is treated with a dichromatic dye.
 10. Adisplaying device comprising a liquid crystal cell, and the polarizingplate of claim 9 being provided on one side or each side of the liquidcrystal cell.